Ой, жалость какая! Кажется, у меня совсем нет совести. Что будем делать?
Some complications you might encounter (you can skip to point 3 if you did not encounter any):
If 0 NiNode (top line in Block list) of the 2 nifs you use has different names, you will get the message "Failed to map parent link NiNode ". Simply rename 0 NiNode of nif2 to match that of nif1 (again, using the Txt icon in Block details top line), and try copying the part again. No need to save changes to nif2.
Sometimes your nif1 will be missing a bone that nif2 requires. The message will be "failed to map parent link ". In Block list of nif2, find the needed bone, right click on it, Block/Copy branch. In Block list of nif1, right click on 0 NiNode, Block/Paste branch.
Back to the nif2, the bone you just copied is still selected. Click the Txt icon in the block details top line and Ctrl-c the bone name. Back to nif1, the bone you just added is still selected. Click Txt icon, paste over the name (it is crucial that the bone is named exactly the same as it was in nif2). You should be able to copy the part you want now.