Poser Pro 2012's library already supports combining different file type into a single subfolder structure.

I haven't used an external library manager for Poser but I've heard good things about some of them, particularly the one recommended by ghostman above.

For me, the PP2012 library does everything I need: configurable, freedom to combine different file types in a folder, a 'favourites' system to further organise the library and support for metadata to make searching easy and powerful. It's far more powerful than many people realise...

Free stuff @ https://poser.cobrablade.net/