Well, in home PC version, you can add a custom exercise much in the same way that you can add a custom food. It's a nice feature, but it might not be worth the $20 alone to you to do, I don't know.
In the online version, it seems to me that the closest one can get is either "Health club exercise, general" which is calculated at 5 calories per minute, or perhaps "Stair-treadmill ergometer, general", which is given at 6 calories per minute. I think that if you have an idea of how many calories per minute you are really burning (and finding this might be harder than you think, calorie counters on machines tend to way overestimate the calories they burn) your best best would be to search until you found any random exercise that has the same value. Using an elliptical is not very much like "Rowing, stationary, 200 W, very vigorous effort", but if you are in fact burning off the 12 calories a minute this exercise computes at, then does it matter how the exercise is labled?
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