You want to add a custom profile field that your users can modify themself? And then auto-populate it with date depending on their behavior in your community? As an example: A user is always active in the "cookie forum", so the empty profile "I like  cookies  coffee   hot chocolade" in your café community will be "I like [x] cookies  coffee  hot chocolade". Thats absolutely possible.
What should trigger a "Callback"? It would be better to run a script once a day auto-populating new data, or execute the script when the user reaches a point where you can say: "okay, let's fill out this profile field".