By saying they will run by themselves, I assume they will run with each other but not strange pack mates. I would separate them from each other and kennel them with some of your hounds so as to let them get accustomed to them. Then I would take 1 of them with 1 of yours and let them start a rabbit. If they will run together feed in new pack mates slowly til the new one isn't bailing out of the race. Then I would repeat the process with the other new one. They may not trust your pack yet as they probably have only ran with each other.
I acquired a set of dogs the same way and they wouldn't pack or hunt with my dogs at first. I seperated them from each other, put them each with two of my other dogs and ran them. After about three weeks I started running all six together and they were fine. I had them for eight years the oldest was twelve and she died of old age this spring. Her daughter was lost in a wind storm and never found. In that area I would guess someone picked her up.