Right now, there is no import/export. It’s a known useful feature, but the devs have no time to work on it (I’ve been following all the optimization work they’ve been doing on github, I don’t know if they sleep). You’ll have to start over atm, sorry.

Nope. You can subscribe/post/comment on any community on any instance. There is one small seam though: if you're the first person to subscribe from your instance, you need to put in the full URL of the community (https://lemmy.ml/c/gaming, for example) to pull it into your instance.

After that, everybody on the same instance as you will see it when searching for communities just like it was local.

EDIT: Oh, forgot to mention: make sure the search is set to "All", not "Communities" when you do this.