OK, now I have imported connections from Diaspora* by hand as long as the people were either in offline or online aspects and I had some sort of idea who they were.
Sadly it seems that most of the people were very inactive, but I imagine that I won't have to do this again thanks to nomadic identity and at most I would need to figure out how to tell Diaspora/Mastodon connections that my account has moved and force Hubzilla to move the connected location in the distant future.
Adding connections was very unfriendly though. For Mastodon I am trying to claim knowing more than Hubzilla knows, maybe it will give me a secret button for mass-importing IDs.