Just to add to what Acadia said, when you click the "+" in the library pallet, the box that comes up will have a button "Selct Subset". That's where you can choose which parts of the pose to include. Un-checking "UNIVERSE" will clear all the checked body parts, which is sometimes handy if you only need to include a few parts.

If you want to use only part of a stock pose you have on disk, you can lock the actors that you don't want the pose to apply to, then after applying the pose unlock the actors (same trick works when applying a walk cycle). Having utility pose files to lock/unlock whole sections of the figure (eg legs+feet+toes, arms+hands+fingers, head+neck+eyes) can help when using this method.

Pose files saved as uncompressed pz2 files can be edited in a text editor to exclude various channels or actors. Once you get used to editing pz2 files you can even add various things like scale data, or data for the Body actor, which Poser does not include when saving pose files.