You can do this with graft points. From the new repo:
git remote add old /path/to/old-repo
git pull
Now, to do the graft and make it so that there is one single tree of commits, edit .gits/info/grafts
00c6d1bc0d3e4cd4bceef5966aa6478ae2c96a5a a936b291fd3cedb9a964a6d05b3c330ee310fe
You will have to change the hashes. The first hash is the initial commit of the new repo. The second hash is the last commit of the old repo.