I tried to use Additional Identities to work around JENKINS-10258.

The plugin adds two text fields - "Identity" and "Realm".

I can reasonably guess that for "Identity", I'm supposed to put in the "Jenkins User Id" shown in the other user's page, since that is the only thing the other page shows. However, it is impossible to guess what is supposed to go into "Realm".

Given that it appears to be impossible to figure out the correct value to fill in here, it doesn't seem like this plugin is usable to merge users together, and a more straight-forward interface where I can choose two users and merge them would be much easier to use.