There is a way, as follows:

First, you need one of the commercial wet textures. Take this into Photoshop, turn it monochome, and play with the levels until everything except the droplets is black. Now copy this as a layer on top of the texture you want to make wet. I can't remember what blending mode you need, possibly screen. You can also adjust the transparency to make the droplets strong or weak.

Then save out the texture as a new jpg.

Obviously this will be for your own use only; the droplets are derived from the commercial texture and mustn't be redistributed.