The Devil's Tongue trait from SCAG states:

> Devil's Tongue. You know the vicious mockery cantrip. When you reach 3rd level, you can cast the charm person spell as a 2nd-level spell once with this trait. When you reach 5th level, you can cast the enthrall spell once with this trait. You must finish a long rest to cast these spells once again with this trait. Charisma is your spellcasting ability for them. [...]

It's pretty clear that the trait enables the Tiefling to cast the two later spells from the trait once per long rest without using spell slots (due to it saying with this trait, instead of with a spell slot).

My question is, could the Tiefling, who is also a Sorcerer, cast them using spell slots during the rest of the adventuring day?

Or to put it another way, are these spells now known spells for the Sorcerer in addition to their classes known spells?