(Disclaimer: Just my own skill level and knowledge here, there may be other ways)

On a basic level, you can do this (just tried it).
There are however a lot of caveats that come with this method.

I tried it with a wristwatch, and here's what I did:

• I added the wristwatch, and then exported the left forearm (to which it is attached) and the wristwatch as a single Wavefront object. Keep everything unchecked in the export options except "Include existing groups in polygon groups" and "Preserve existing material names".
  EDIT: And make sure that you figure is absolutely zeroed before exporting - no morphs, no scales, no anything else!
• Then I deleted the original wristwatch from the figure and reimported the object I had created before.
  A forearm with the wristwatch should load, in the same place as the figure's forearm.
• Select the figures's left forearm, then from Poser "Object" menu select "Replace Body Part with Prop".
  The original forearm will be replaced with your forearm with the watch.

That was the easy part.

BUT: =====

Because the watch is now a part of the forearm geometry, the bending of joints will affect the watch too!
This might not be a problem in case of the strap, but you certainly don't want the casing to bend and deform with the wrist joint!
So you'll have to edit the joint zones in question, and traditional rigging would lead you nowhere here. You'll have to use weight maps.
Like this, I think you could weight map the watch casing in a way that it moves along with the strap, while keeping it's rigid shape. Interesting challenge, and probably worth looking further into.

Yet, there's a much bigger problem:
Because of the different poly count, all morphs that were in the original forearm are lost!
You can bring them back if you load another, identical figure which still has it's original forearm with the working morphs, and just use the "Copy Morphs From..." Poser Menu Command to copy the forearm morphs into your modified figure.
BUT (there's a lot of "but", I know...):
Poser does copy the morphs (in sometimes questionable quality, so check each morph for irregularities after copying!).
But it does not copy the dependencies, if there are any. So spinning the "forearm thickness" dial in the BODY actor won't affect the (new) left forearm morphs.
You can remedy that by either setting all morph dials in the forearm manually, or you must edit the cr2 file and recode the dependencies, one by one.
Quite a job to do...

Yet, there's another "BUT": Though it's improbable in the case of a forearm which is rather neglected by *Third Party Morph Injections" which use pmd files, it might become an issue in other body parts:
Because of the differing poly count, those morph injections won't work on the, your modified, body part, so it starts all over again: Load a figure duplicate, inject the morphs there, copy them to your new figure, resurrect dependencies...

How all this could work with clothing, I have no idea. But given the amount of work necessary only for that stupid wristwatch, I doubt that it's a practical way to go at all.
Maybe in a future "PP2525, if man is still alive" :D

HTH
Peter