1. Open two models, each one in a different Nifskope.
2. Right click on the part of the object you want to copy.
3. Click Block > Copy Branch.
4. Then in your second model, the one you want to combine it with, go to the Block Details.
5. Look for the Scene Root (It should be at the top), right click it.
6. Click Block > Paste Branch.
7. It should appear now but you need to know how to change its size and position. For this see Moving a Model Along the Axis in Nifscope