I know I can do this by copying each element myself, but is there a method that does it for me? I tried mat2.copyTo(mat1.row(0)) but that doesn't work.
mat2.copyTo(mat1.row(0))
Try
destMat.row(i) = (sourceMat.row(i) + 0);
It's not very pretty but it gets the job done. Also see this. Read the comments on Mat::row