Hi Sarah, congrats on your progress so far, and welcome to FitDay.
Logging your own calories burned isn't an option at this time, though supposedly an option to customize an activity is in the works.
For now, what you might consider doing is finding an exercise that's similar, then tweaking it to match your calories from your own monitor. Try to keep the duration the same, otherwise it throws off your base metabolism count by messing with that time.
I have a heart rate monitor and what I used to do to get the calories to match was, for example, put in 25 minutes of walking 4.5 mph and 15 minutes of walking 4.0, or 40 minutes of tennis, singles, and 20 of tennis, doubles, just to get the calories to work out. If you're lucky, you can pick one activity and it will be pretty close.
I'm waiting for that upgrade, too!