If you already have an active goodybag you can not top it up with another goodybag for extra minutes or data as you can only have one active goodybag at a time. You would have to use credit on PAYG for additional minutes or data (at the reduced rate of 2p/Mb) until the active goodybag expires then you can start a new one.
It is not possible to end an active goodybag early in order to start another.
There is a proposal in the ideas section to finish a goodybag early in order to start another. If enough people get behind it then giffgaff may look into implementing it.
Go here http://community.giffgaff.com/t5/Submit-Great-giff