Can I assign a unique datasource_id in a subscribe form?

[Dabbler]

Nothing in the manual that I see.

I want to track the source of my subscription sign ups. For example, my subscriptions will be coming from:
1 - CSV imports
2 - manual additions
3 - double opt-in form "a"
4 - double opt-in form "b"
5 - fast subscribe forum "c"

I particularly want to track the different subscription forms so I can evaluate how effective they are.