Si tu n'as pas la priorité, non ; sinon tu peux.
Oui, ça parait un poil étrange, mais bon :
Citation :
605.3a A player may activate an activated mana ability whenever he or she has priority, whenever he or she is casting a spell or activating an ability that requires a mana payment, or whenever a rule or effect asks for a mana payment, even if it’s in the middle of casting or resolving a spell or activating or resolving an ability.