Update January 2, 2016: Since the last update, there is full support for reserve lists in Athletics.app.
You can set this up through the wizard, step 7 "online registration settings. Then in step 7b you can set how many athletes are allowed and how many reserves.
See also this answer:
https://www.athletics.app/faq/?qa=111/automatisch-reserves-bijhouden-1e-2e-3e-reserve&show=111#q111
Please note that if you work with reserves, the online registration does NOT automatically close. People can continue to register until the registration date has passed or until you manually close the registration for a specific group. See also this answer:
https://www.athletics.app/faq/?qa=340/kan-ik-inschrijving-ook-deels-sluiten-als-met-reserves-werk
This has been automatically translated into English