If a spell requires you to give a unit temporary, can an already temporary unit be chosen as the target for the spell?

Yes, an already Temporary unit can be chosen as a target for a spell that grants Temporary.

According to Rule 816.2, multiple instances of Temporary are redundant. While the unit will technically have two instances of the keyword, they will only trigger once as a single game event (Rule 816.2.a).

Since there is no rule preventing a unit that already has a keyword from being targeted by an effect that grants that same keyword, the targeting is legal. The unit will simply remain Temporary and will still be destroyed at the start of your next Beginning Phase.