If you don't mind doing a little manual editing of the PZ3 file you can do this it a text editor.

NOTE --- Always do this on a copy of the file. (Just in case you make a mistake.)

Open file with text editor of you choice. (It needs to have a search feature. I use textedit on my mac.)

Search for, - addActor - It will find the list (of all items in that PZ3 file) that Poser uses for the drop down menus.

Cut and paste the relevant items in this part of the file into the order that you want. Save file and test, then rename back to the original and overwrite old file.

(Caveat - In this file the 'name' used is the 'internal' poser name, not the 'display' name. But for props this is usually the same.)

Editing like this always carries a risk. Just make sure you always do it on a copy of the file, so that you can always revert back to the original.