I believe you have to submit your own website to this one for manual addition to its index

Yep this should do the trick! I see it’s been 2 weeks so: how did they turn out?

If i understand you correctly: you are not limited to the small instance dropdown list in Jerboa, you can just type your own instance name in there to log in to it. That’s what I did at least. If you mean more instances in general should be available in Lemmy: agree :)