You open a model in mdlvis, delete all mesh except what you want pasted into other model, save, just in case, select the mesh, click copy button, open a new model, click the paste button, select that pasted mesh again if it wasn't selected, and press 'detach as new geoset' or just leave it if it uses the same texture as the geoset you're pasting it into. then you press 'F3' go to the 'bones' tab, select the pasted mesh, select the bone you want to attach it to, and press 'reattach vertices' button (the other attaching one kinda does bad things). after that you just save the model as separate mdx, and can tinker with it more in magos or in mdlvis