if RepoB was done from RepoA history (as a true fork: starting from RepoA, and changing everything in it), then importing RepoB history in RepoA coud be done (assuming the main history to merge is on the master branch) using "ours" git merge merge strategy:
cd /path/toRepoA # local clone
git remote add repoB /url/to/repoB
git fetch repoB
git checkout -b masterRepoB repoB/master
git merge -s ours master
That merge will make git consider master as merged to repoB/master... while keeping only repoB/master content (no files from repoA/master is in the result of that merge).
See also "Why would one use “git merge -s ours”?".
Then:
git checkout master
git merge masterRepoB
That will fast-forward HEAD from master to masterRepoB, making that master branch identical to repoB/master (masterRepoB)
You can then push master to /url/repoA: its content will be the one from repoB.