chris,

Yes, it's possible, by using the grouping tool.
(I'm not sure when this tool was introduced, certainly it's in Poser 5 and later).

Referring to the pic above...

1 - Select the head.
2 - Change display Style to "Hidden Line" (otherwise you'll be unable to see anything much).
3 - Open the Grouping Tool.
4 - Start a New Group and give it a name (eg Eyebrows)
5 - (the tedious bit) carefully select the polys you want. They'll change to red. I didn't do anything carefully here, just selected some representative polys.
6 - Hit Assign Material and give it a name (eg Eyebrows - doesn't matter if it's the same name as the group or not).

Exit the grouping tool (and give deep sigh of relief).

When you enter the Mat room, you should now have a new material available. Materials added later should override existing materials. Not an area I'm clear on, but it's not caused me any problems so far..

Hope that helps,

Cheers,
Diolma