Usually, I use a "Toon" render or just a standard render in Firefly and I do my renders on transparent backgrounds. Is there a way to render over a transparent background with "sketch" mode? 

Maybe there is, I just save out a black and white copy and set as a multiply layer in an image editor, then you're free to do all kinds of non destructive additional manipulations. Here with a color layer and a canvas texture layer...

If you want to isolate just an object rendered in sketch onto a transparent background, you could just cut or erase out the opaque background or render over flat white with no ground and again, multiply. There's several solutions I think. Save out the target layer, after isolating and ridding the BG, deleting the rest, as a png without flattening (which will flatten to an opaque background). You'll end up with your sketch rendered focus on transparent background file.

Took a moment to play around with it. You'll probably not want to render over a totally white empty background because of the annoying fringing you'll get when you composite your work. For this I turned off the ground, loaded a very scaled up plane to act as my background, set that to totally black diffuse with no specular. You can render with the background set as black in the scene but this causes the sketch renderer to effect the foreground object, the focus, in a particular way. As you probably know, the 'Render Over Black' in the Sketch Designer itself also behaves not as typical but results in a negative render, sort of a scratch board effect. That's not what I'm looking for here. Overall I want to control and produce a black and white line layer and a color layer for maximum flexibility in compositing into an object on a transparent background. It's a few steps but each element takes literally seconds to produce so not a big deal.

First I turn off the ground visibility, load a backdrop plane, and make it flat black. For the sketch renderer I imagine you can just leave the ground on and make that all black. I personally create a lot of smooth shaded lined preview render images and a background object set to black (other than the ground plane/construct) is a matter of habit for me for various reasons relating to wireframe opacity and fill layers with preview renders.

I used a a woodcut setting for the line work in the sketch renderer. It's practically impossible to render out a completely solid black background (AFAIK) with the sketch renderer without seriously influencing the foreground object focus. I played around with the background settings as much as was feasible to produce the most dense effect...

For my final image I want a very strong black holding line to surround my focus. For this I turn to the comic preview options, Geometric Edge Lines, to produce the holding line. A magic wanded stroke layer in composite would suffice as well. I just produced the layer here in Poser. I turned off my background plane, changed the background to black, converted an infinite to diffuseIBL (you could simply flood the object with ambient also), and flooded the scene with diffuse. It's really just a few clicks. The holding line image...

Finally a quick preview image color layer... with a bit of shading

Over to the image editor I assemble my layers, do a couple magic wand selection cuts and blending modes (multiply/linear light)...

Merge visible (not Flatten) so I'm left with my object on a transparent background... save as png.

As an aside, there's probably an easier method that will save some steps... in particular there's a forum member, HartyBart, who's very knowledgeable about these types of Poser renders. Perhaps he'll chime in with a more streamlined workflow.

The problem with Poser Sketch renders, amazing and fast though they can be made, is: i) there is no transparent background in Sketch renders; and ii) the outline of the subject often 'wobbles' slightly due to the Sketch effect, which means it's then difficult to make a fringe-free cut-out from the Sketch render (by later using the Comic Book mode real-time render .PNG or Firefly .PNG as a mask in Photoshop).

One way to do it is to render on white, then use a 'knockout white to transparency' Action in Photoshop. https://www.deviantart.com/mutinate/art/Knockout-All-White-for-PS-889467682 Then manually resize until its edges more or less fit just within the edges of a standard .PNG in the layer below. Mask the lower .PNG, and fill with solid white.

Alternatively you can sketch a straightforward render in a filter, although in more limited ways. The only ones worth using for that are the standalone StudioArtist v5.5 or higher (vectorises, then does the effect) and the Photoshop plugin Redfield Sketchmaster 2018 or higher. Both are expensive.

The problem with Poser Sketch renders, amazing and fast though they can be made, is: i) there is no transparent background in Sketch renders; and ii) the outline of the subject often 'wobbles' slightly due to the Sketch effect, which means it's then difficult to make a fringe-free cut-out from the Sketch render (by later using the Comic Book mode real-time render .PNG or Firefly .PNG as a mask in Photoshop).

One way to do it is to render on white, then use a 'knockout white to transparency' Action in Photoshop. https://www.deviantart.com/mutinate/art/Knockout-All-White-for-PS-889467682 Then manually resize a standard .PNG render in the layer below it, until the Sketch more or less lines up with it. That, if then filtering these flattened layers further, will sometimes do the trick.

Alternatively you can sketch a straightforward render in a filter, although in more limited ways. The only ones worth using for that are the standalone StudioArtist v5.5 or higher (vectorises, then does the effect) and the Photoshop plugin Redfield Sketchmaster 2018 or higher. Both are expensive.

Forum software seems to have duplicated the posts. The second later one gives the correct advice.