You can’t merge a repository into a branch. You can merge a branch from another repository into a branch in your local repository. Assuming that you have two repositories, foo and bar both located in your current directory:
$ ls
foo bar
Change into the foo repository:
$ cd foo
Add the bar repository as a remote and fetch it:
$ git remote add bar ../bar
$ git remote update
Create a new branch baz in the foo repository based on whatever your current branch is:
$ git switch -c baz
Merge branch somebranch from the bar repository into the current branch:
$ git merge --allow-unrelated-histories bar/somebranch
(--allow-unrelated-histories is not required prior to git version 2.9)