Attached Link: http://www.renderosity.com/messages.ez?Form.ShowMessage=487037

If you have Pro Pack installed, Poser will not convert to .bum, even if you have it at gunpoint. You'd have to install a second copy of Poser, without the Pro Pack upgrade, in a different directory, and use it for .bum conversion.

You could also try running an emboss filter on the .jpg in your favorite paint program, saving it as a greyscale .bmp, and then renaming it to .bum. The file formats are almost identical. See the thread above for details.