Not directly (As far as I know). Some spells are unit only where you have to do that each time. Some you only have to target one person and it affects the whole army (like Tireless March).
You can, however, edit the XML to create a version of the spell that you cast on one person and it affects their entire army though. For the most part you only need to change AdjustUnitStat to AdjustArmyStat