There's no such thing as a "single engine land" instrument rating. It's an "airplane" instrument rating. I presume you have a pilot certificate with single engine land rating on it already.
To get an instrument airplane rating is the same whether you have the helicopter one or not. Obviously, you get credit for the aeronautical experience for things that aren't specifically marked "must be in an airplane" in 61.65. Otherwise you don't really get any credit for having a different category instrument rating.