The .bum is a strange, poser exclusive format. When a texture that uses it is loaded in Poser 5 (and, I guess, P6) it's plugged into a special "gradient bump" node, different from the standard bump plug, as it doesn't behave as an average b/w bump map.

In fact when I switched from P4 to P5 I had no problem loading P4 metarials. They just didn't look so good as P5 ones.

I don't think the problem is P6 being unable to load P4 materials, as it has the "gradient bump" node too.