Thanks to all for the advice. For now, I've figured out a workaround solution. I started a new drawing, drew a rectangle for the material, and imported it with no problems...just like it should work.
I did that to establish the new material in my materials library. Then I opened to old file (in which establishing a new material didn't work) and brought in the existing material with no problems.
Something about that file just prevented new materials from being created.
Thanks again for your help.