Does anyone know of  a way to convert a maya .mb file to a format usable in Poser with joints intact?

Thanks!

I'm not really aware of any software that can import the .mb format I believe it is proprietary to Maya. and unfortunately the type of rigging that is used by most of the high end software is so incompatible with Poser rigging that it's very unlikely to translate usably if if you could even import it at all to poser.

And suppose I had access to Maya.

Is there an easy way to export it?

Yes but It won't do you any good for the joints. As I said Poser joints use a far different system than other software, Maya uses weight mapping which doesnt translate to falloff spheres. so you would have to export as an .obj file and than rig it completely in Poser.