Take for instance, a level 5 (standard) tiefling warlock. Would you be able to cast your Darkness and Hellish Rebuke spells with a spell slot? Both spells appear on the warlock spell list and could be taken as known spells by a warlock. But let's say you didn't take them as known spells with warlock. You can cast each of them once per day with your Infernal Legacy trait without expending a slot. The trait says (emphasis mine):
You know the thaumaturgy cantrip. When you reach 3rd level, you can cast the hellish rebuke spell as a 2nd-level spell once with this trait and regain the ability to do so when you finish a long rest. When you reach 5th level, you can cast the darkness spell once with this trait and regain the ability to do so when you finish a long rest. Charisma is your spellcasting ability for these spells.
Since it specifically says you can cast it once with this trait are you able to cast it with spell slots without having to take it as one of your spells known as a warlock?