There's no way to add a custom activity as yet. I don't know if they're working on this for FD 2.0, but you may want to ask this question in the Beta sub forum. For now, what you can do is see how many calories are burned with the monitor, and then change the time for the canned activity to correspond to that number.
To clarify, if your monitor says you burned 600 calories running, but running for 35 minutes using FD only displays 525, then change the TIME for the FD activity so that it then displays 600 calories burned.
Hope this helps.
Think of food as fuel for the body instead of feeding emotions