Good job on thinking up this concept, however this is an old concep that has been used countless times and has been thought up by many people.
This is actually one of the basic spell methods along with the building units method. This is the dropship spell method.
You could consider using a gateway temperary scroll system that lets you buy spells from a shop? Or liftoff a shop in-map that scrolls through the buyable spells and uploads into the dropship? Having a demostration for the spell/skill if you choose the help option on the gateway system... Tons of different ways to spice this old system up!