You can do this simply by setting the correct speed with the .speed xxx command and using auto-speed. You just need to take a minute to find the correct speed and note it for future reference.
I know what he is asking for, it's used in modern autopilots (i'm thinkin airliners), VS hold (or maybe VS select?) if i remember correctly is the name of the ap mode. I think it would be of limited usefulness in the game, since we're mostly dealing with oldschool prop planes where the performance is so different thru different alts, and we aren't dealing with air traffic control who like to see predictable vertical speeds in order to separate IFR aircraft from each other.
however, .speed (alt x) wont work to maintain a VS. .speed commands pitch to maintain a speed. likewise the auto-angle (ctrl x) just commands pitch to maintain the pitch attitude, wont maintain a VS or a speed -- the VS and speed will go wherever they go.
the OP's wish is to have the autopilot command pitch to maintain a vertical speed, and let the speed go wherever it goes.