Unfortunately there are no logic way to do this. However a way you can do it is to export the final frame of the simulation as an obj file (Make sure you only select the cloth). Then import it again instead of the current cloth. Now the default shape of the cloth will look like the last frame and you just have to setup the simulation again, meaning just replace the old cloth with the new cloth you imported in the cloth room, all the settings should be the same. Also make sure that you reapply the textures after import.

It would be a very nice addition to Poser if you could actually set an initial starting frame, hope they add that at some point.