Update:
Set UV to origin 0,0,0 I misunderstood while reading the tutorial, wanted to relax the mesh UVs, set the UV-boundary enclosing the rest of UV-s align it to mesh size, etc.. All this isn't needed.
In XSI:
1. Create any mesh you like, leave a hole at bottom as a loop (attachment-place)
2. Give the mesh an UV property.
3. Select UV verts, enter 0,0 into UV-coordinate fields: the whole UV set collapses into one point to U,V. coordinate: 0,0
4. Export the mesh for use in meshmixer
5. In Meshmixer: import or F it loads perfectly
6. From main menu select Actions---Convert to part
7. Meshmixer converts the thing to a part: changes color
8. Load your main model into meshmixer: a human or a big robot, etc..
9. Attach the newly created part to your main human mesh model.
10. Export to OBJ from meshmixer: works perfectly
11. Import into XSI: model loads perfectly, review the new combined mid/highpoly
For aesthetic reasons a midpoly mesh needs to be created in XSI to retain mechanical details in meshmixer after mixing - and because meshmixer will not have a quadrangulate function in the near future and it is not really needed as it is clear now that a retopo of the meshmixed model must be made at the end anyway.
So a very very nice meshmixer04 we have here. Congratulations Ryan!
OBJ import, meshmixing, smooth brush, export of the created model to OBJ, then importing it to XSI is working perfectly.
Ready for going commercial, i would say!
