If Severin has the power to take away the demon within, maybe he can put it in, too.
There is no indication that Severin can do that, and even if he could why would he do it to Buffy? He wanted to take her powers, not give her someone else's. And even if he did it for some unknown reason, it would be silly even by S9 standards for this demon or whatever to show up on a pregnant test. I mean, I doubt that demonic pregnancy tests are available just yet, even with demons being outed.