Organization is key. Have folders for: the model you want to graft onto the MP Character, the MP Character's model you want to use as a reference + the "model name FULL.skel", and a folder for your final product
Again, in your MP Model folder, have the xxx FULL.skel file alongside the model you wish to use as reference in there (and a .otd file for textures to show up in 3ds Max. Completely optional)
In the folder for the model you're importing, all you really need is the .odd file + its accompanying file folder from the OpenIV export
When in 3ds, import the new model first so you can have it ready for attachment/editing. Of course the order don't really matter, just my process.
Go ahead and delete ALL bones/nodes other than the "_high" model as such
Once that's done, go ahead and import the MP Model and select that model's "_high model"
Afterwards, go into the "Modify" tab on the right-most window, and go into the editable mesh. Use any of the 5 selection modes (vertex, edge, element, etc) and select the entire "_high" model using either ctrl-a or dragging the mouse. Then attach the selection to the importing model and hit delete.
Export that into your Final Product folder, and that's it. Keep in mind for this example, feet_010 was my MP Model and feet_000 was the model I'm importing