On a normal saveslot eligible for boosting you just need to hold SPACE for a few seconds and a new, boosted level-45 saveslot is created.
For a hacked saveslot eligible for boosting the boosting option seems not to be available.
With "eligible for boosting" I mean: "Blood Drive" present day quest is done. Boost-to-level-45-now-available message appeared. Starting quests for both DLCs are available. Free saveslots.