You basically have 2 options in Poser - dynamic and conforming clothing.

However, the original model has to be modeled with separation where you want things to open/close.  So, if you want to unbutton a shirt, the front has to be modeled in 2 pieces with a seam in order to morph the open option.  Pockets have to have an opening (and a liner inside the pocket?), etc.  The bottom line, though is the model has to be created with this end result in mind.

When you create morphs, you can move around polygons (i.e., open/close) but you cannot alter the count though deletion or addition.  Doing so will create a model with a different vertex order than the one you are trying to morph and fail.  You should move the existing polygons to create your morph.

Does that help answer your question? If not, perhaps you can ask more specific questions I can try to answer for you.