Hi Lizou Gilinsky. There is not a way to manually add a workout that you did not do through Grokker. If you have a FitBit or Garmin, you may sync your device to Grokker. Those workouts will be tracked to your calendar (Grokker will only store the minutes of your workout).
I don't have a device, but when I go to the gym, I want to have my time on the treadmill or eliptical machine counted. I've loaded the FitBit app to my phone and synced that with Grokker. When I go to the gym, I track my workout through the FitBit app as either a run or walk. Those minutes are then synced to my Grokker account and will show up on my calendar as a walk or run for the day.
Does that help? Please let me know if you have any more questions.