IrfanView can open .bum files.

It actually will give a notice, saying that those .bum files are actually .bmp files and offers to rename them for you.

(So you probably could also just rename them manually, so that newer versions of Poser can handle them)

IrfanView can batch convert them, too, into a file type of your choice.

If I remeber correctly, I think Poser 5 also auto-converted .bum files into .jpg's?