Treat the payment as "Check or Money Order Entry"
As far as I know, there is no way to initiate the subscription from he admin page. This is something the user needs to initiate. You could enable the "Check/Money Order" option for each subscription. And then when you receive the payment from the user, Update that item status from "Pending" to "Active," and set a date for expiration.
If the user already has a subscription item within the "Transaction" page, edit that to set the time you'd like that subscription to last for. The subscription will continue for as long as you have set, regardless of the type of payment you allowed them to select.
If this is only a one time thing,
1) set your subscription group up to allow the users to create a "Check/Money Order" subscriptions.
2) Tell the user to choose that option. (or "Become User" and do it for them)
3) When its in there, edit "Pending" transaction with the expire date.
4) Disable the "Check/Money Order" option from the group. If anyone manages to sneak in there while you're doing this, just delete the transaction, or leave it as "Pending."
You'll get a Private Message (also by email, if enabled) notice with each subscription request/change/update.
To answer your question more directly -- I dont believe there is a way within the current Control Panel to add a time limit to a subscription if that user never requested a subscription first. Because its the subscription which determines the time.
NOTE: Some of the subscription menu terminology has been updated for UBB.threads
7.6.1 (not released at the time of this post) to avoid confusion, and add more clarity between "Groups" and "Subscription Groups."http://www.ubbcentral.com/forums/ubbthreads.php/topics/259894#Post259894