OK so did the original shirt have a collar? If not and you added more polys to the new obj then you will have to treat it as new clothing prop which would entail rigging it to the character model one other possibility and there is a good video tutorial where you could treat the collar as a morph instead this tutorial is for Daz but many of the steps are similar to poser https://www.youtube.com/watch?v=npue0bHmjkk another set of tutorials that I would highly recommend by Tony Vilters are these and you might find them actually more useful for what you might be trying to accomplish https://www.youtube.com/channel/UCiiFNpMB2ENEiUuctAUObmg

I hope they help

Lobo3433

Blender Maya & 3D Forum Moderator 

Renderosity Store

My Free Stuff

Facebook

Twitter

Renderosity Blender 3D Facebook Page