Inbreeding is not an issue.
Lab colonies have sometimes been closed for decades. What you do is get your initial group and begin breeding. You eliminate any undesirable traits by culling the parent(s) as needed. With any luck and a strict culling ethic, you can shape your colony to what you want to produce(for feeders, you want large litters, quick growing babies, moms that are sturdy and able to breed without getting weak, and rats that do not bite, maul litters or chew enclosures).
Once you have a viable colony, you simply hold back new stock from the best you have. If you have a mom that throws huge litters, save some of her daughters. Don't worry about breeding daughter to father, it will only highlight any issues you would want to cull out anyway and it will also enhance those traits you are breeding for.
If needed, you can add in rats. I would only add in rats if your stock is not up to snuff. If the two males you get are aggressive for instance... cull both, get new. Be prepared to feed off every youngster from unsuitable parents. Keep records so you know how to cull a line if needed. Eventually you'll have a closed colony that will produce well.
Never ever let rats run out of water. Keep them clean and feed them good food. And CULL any rat who chews the bin. At one point I culled entire bins that had a chewer. One chewer doomed the whole bin. Now no chewing. Same for biting rats. No biting rats now.
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