From the picture I would say this is a male cat, not a kitten. If you look closely, it seems his lower lip may be a bit swollen and he seems to have a bit of fur missing from his nose.
I would definitely NOT let him near your cats at this time. If you are truly wanting to keep this cat, then I would suggest that each time you put food out, bring it a little closer to the basement door and if you can, just bend down and watch him eat. Talk to him in a soothing voice. You have to gain his trust first. Once that is established, take it from there. Be patient, it may take time.