Copying or merging wiki user accounts

There are several use cases where user accounts need to be copied from one wiki to another, e.g.

Merging two wikifarms

Wikifarms store user accounts in the metawiki database, which is shared by all wikis in the wikifarm. If the wikis of a wikifarm (or a selection thereof) need to be moved to another wikifarm, then a merge of the user accounts becomes necessary.

Moving a wikifarm to anothers server

Strictly speaking, moving a wikifarm to another server can be done by copying the whole databse. In practice however, the new wikifarm will often be a separate branch of the original wiki, so a merge becomes necessary.