If you have the contribution permission on the group, you can convert eligible one-time donations to recurring donations when viewing an activist's record, in the donations section. (Ticketed event purchases cannot be converted, only donations.)
Click the convert button to bring up a window where you can choose a period (weekly, monthly, etc...) and a start date (starting with tomorrow). Confirm to convert that one-time donation into a recurring donation.
Note: Be careful with this feature. You must only convert donations in this way that activists ask to be converted. You are responsible for any chargebacks (at $15 each) resulting from disputes your activists have their your charges.