If you haven't already found it here is a useful and not-too-expensive tool:

http://web.t-online.hu/karpo/ [web.t-online.hu]

I only discovered the other day that it converts from say 3ds to houdini, it supports .bpoly .poly .bgeo .geo and many, many more.

From that website, it says that it only saves Houdini ASCII .geo flies.