I would say yes, I believe it would be possible. I also believe it would be much cheaper and easier to fit the SRT6 with an E55 Kompressor engine since it's easier to come by. With some modifications, it will be just as effective as the SLR engine; I believe they share the same bottom end.

Also, keep in mind, the SLK55 comes with a V8, where as the SLK32 (the car I believe the SRT6 is based on) comes with a V6, so there may be some extra modification necessary to get the larger engine to fit.