Take note of the following:
I’ve begun my experience, but plan on accumulating more hours before dental school begins. Can I include those?
In the experience sections, you may only document (in hours/weeks, etc.) the time already completed. Once you submit your application, you may not update your hours. However, in the text box under “duties,” you may clarify your expected time commitment, and send any updates directly to the schools to which you are applying.
I haven’t begun my experience yet, but plan on doing so soon. Can I report that?
No, you cannot report “planned” experience. Once you have accumulated the experience, however, you can add new experiences to your application, even if you have already submitted it. Please note you cannot edit experiences which have already been entered. You may send updated information directly to the schools to which you are applying.
Supporting Information
Add the activity once you've started through AADSAS to update your app and you can email schools notifying them that you've updated your app. If you were to pick one, update through AADSAS because schools pull your app from there and will be notified on their side when updates are made.
While some schools may appreciate you taking on new activities, I would not think that it'll have a strong impact on your application because if you report it shortly after starting, you're writing about what you anticipate to do but what actually happens may be completely different.