does XSI support dxf, 3ds, or IGES? That would work for geometry.
for basic translations (Move, Rotate, Scale), then you could probably write simple scripts to dump the translations into a form readable by max. How complex is your animation? Are hierarchies involved?
the usual suspects "polytrans" by "okino" and "movetools" by "Lambsoft" seem to not support XSI or have disappeared.
anybody else got any good ideas?