As the challenge organiser, you have the ability to add activity on behalf of your participants. You can only do this is the user is in a team and the challenge is currently active.
To do so, first log in to your admin area, then go to the Users section.
Now select or search for the person on whose behalf you'd like to add the activity.
Select the user, and go to the "Distances" tab on their profile page.
Finally, use the "Add activity" form at the top of the page to add activity on the user's behalf. You can select the activity type, the unit (steps / kilometres / miles) of the activity, the activity's date and the activity amount.
Once added, you should see a "success" message and the new activity should appear in the list of activities below.