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?
Questions I am not asking
I'm not asking "Can I cast a spell that isn't on my class' spell list?".
- The general rule is that characters only have access to spells on their spell lists. The Devil's Tongue trait adds a specific rule that says you can cast this spell with this trait (which means you have access to it despite it not necessarily being on your class' spell list). This question is the next logical step, given that you have access to this spell from can you also cast it using your class' spell slots?
I'm also not asking "Do I have to use spell slots to cast these spells?"
- As mentioned in my question I regard this as obvious that you can