Up to release 3.2 it is not possible to "prolongue" scheduled transactions.
There are to possible ways:
1. Allow copying of a scheduled transaction and using the data as a template for the new scheduled transaction but with new start/end dates
2. Just allow (eg. set "allow edit transaction" flag under preferences) editing the "end date" on current transactions
For handling reasons you may ask the user before every automatically booked transaction with ok / cancel buttons if he REALLY wants the transaction booked.
On the other hand, using the cancel option would make a "execute now" function on a scheduled transaction helpful.
Best regards and thanks in advance