it is:
go to the instance locally, say you wanna sub to https://kbin.social/m/Marvel, you'd want to go to this url: https://lemmy.ml/Marvel@kbin.social, then in there you will find a Subscribe button. Just click that.
If you wanna have an automated way of switching from kbin to lemmy, you can use bookmarklets like this:
https://lemmy.world/post/48909
or this:
https://lemmy.world/post/48816
i won't guide you on how to use a bookmarklet, so google that if you want. you can do what you want without a bookmarklet by following the instructions in my first paragraph.