Great problems here... you can use any modeler to do that, even blender. But you need to know hou]w to use it. Other problem is the rigging (the bones for the jacket conform) it will need to be changed a little in Poser you can do this but its not easy (theres a lot of tutorials about creating cloth for Poser), second: the UVmap will need to be redone, just because if you use a stretched mesh the map will be stretched too, so you will need to correct this using UVMapper or the modeler that you will use to change the cloth.
When you need only to do some adjustments to the mesh only use magnets... my advise is search for a better jacket that fits your needs.

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