you can start with a shirt open, and then add the morph(s) to close it. It doesnt really matter , the process is the same.

Crete an obj file and rig it for the character (donor rig).

then model and load the morph(s)

extract the morph data from a new saved cr2 and create the inj pose file. (there are scripts to help do this bit)

Just remember this : do not change the number of vertices or their order in the morph obj. If  you do you get an error when you try to load the morph.

You can make the morphs in Poser by using magnets to deform the clothing, then spawn the morph from that.