Some cows are pretty easy and some more difficult. I have a Jersey that I bought for several reasons, this being one of them(to raise an extra amongst other reasons). I wait until she has calved. When I bring in the "extra" I keep her and the two calves separate from the rest of the herd for a week or so. To get her to accept the "new guy", I put up her calf in a pen, then put her and the "new guy" in a pen right next to that one. Usually I have to tie her at first, with her head by her baby and she is fine. Then I "show" "new guy" what he is supposed to do, let him suck a while, then I bring her calf in with them. After doing this 2 or 3 times she is fine with the new one. At first she may push him aside, but then she takes him. After a week or so, when they are a happy family, she returns to the herd. Finding a way to get the scent of the "original" calf onto the new one does help.
When we did this at the breeding farm with horses, we used to use Vicks on the nurse mare's nose and that seemed to help. Haven't tried it with cows though. Good luck.
Katherine