From what I've heard, wild rats can't be fully tamed, with the exception of roof rats (I think, or could be a different one). Even if a domestic rat got pregnant by a wild rat, and the domestic rat was super tame and friendly, the odds of getting the babies to be tame, are very low, and when/if you do, they will still have wild tendencies, like scattering with even the tiniest noise, biting, etc. Lilspaz has some experience with half wildies. She could help you better.
That being said, I'd NEVER take a wild rat out of its natural habitat to try to tame it. That's cruel, and unnecessary. Plus, if for some reason, you HAD NO CHOICE but to raise a wildie, you can never release it back in the wild. Rats are very territorial, and will kill newcomers to their territory.