Can I play a “charger” spell just to put an extra card into my mana zone?
Yes. For example, Energy Charger says, “One of your creatures gets +2000 power until the end of the turn. Charger (After you cast this spell, put it into your mana zone instead of your graveyard.)” If you don’t have a creature in the battle zone, Energy Charger doesn’t do anything when you cast it. Then you put it into your mana zone.