However, unless you have a fin tab, your fin will be floating in air where the transition is on the -Y plane.I haven't tried this in Openrocket but it imports Rocksim files just fine...
I attach the fins to the body tube in front of the tail cone then adjust their position appropriately.
The downside is that the program likely uses a value for the fin area that is too small. The result being an overly conservative CP location. If you attach the fins to a zero length piece the size of the back of the tail cone it is likely that too large of a fin area will get used. Leading to an optimistic CP location.
The portion of the fin inside the main body tube diameter likely has reduced effectiveness as well. Depending on the tail cone angle and airflow details.