I haven't done much more than observe that this "appears to work", but: you can install the Havok Content Tools for the (free) XSI 6 Mod Tool by installing the HCT for Softimage 6 (to any directory I think), then copying the contents of the 'HavokContentTools' directory over the 'XSI_6_Mod_Tool' directory.
In my case, that was:
- HCT installed to 'C:Softimage', creates a 'C:SoftimageHavokContentTools' directory
- Copy the contents of 'C:SoftimageHavokContentTools' (Application, Data, etc) over 'C:SoftimageXSI_6_Mod_Tool'
- Start up XSI Mod Tool (you should see a 'Havok' entry added to the menubar)
- Open eg. 'C:SoftimageXSI_6_Mod_ToolTutorialsScenesMoreOnRigidBodies utorialEnd.scn', select 'Havok'->'Export'->'Run Configuration'->'Play' under 'Physics Simulation'->Enjoy physics simulation goodness.
Your mileage may vary of course- back up anything important before trying this etc.