Hey there,
I am interested in further developing this tool personally because I am in need of it for my project. (I have a entire library of Havok hkx's I want ported to COLLADA for use with the the Bullet Physics engine).
The source code looks good and but is in need of completion which I am willing to do. I have a few questions:
Your code creates most of the common Havok shapes into Bullet ones. My Havok files contain extra data types like skeletions, animations, bindings, attachments and skins. At present, these data types arn't handled? I would like to add this in but I could use your help, just simple starting braces with more //TODO notes would do. I will use the Havok and Bullet documentation to fill in the rest.
Please correct if I am way off track here!

Many thanks in advance, Sam