That is very odd!!
You can "change" Material names in the grouping tool by Adding new material names and assigning them to the correct group. Then removing the incorrect material.
Out of curiosity, what are the materials named in the MTL file?
I'm asking cause software does not have free will, and won't assign names willy nilly. It has to get the data from somewhere!

(you can also modify the poser files by hand.... which is a great way to irreparably MESS STUFF UP!!! Back up your backup before you begin!)

;>

Andy!