Unearthed Arcana: Revised Class Options includes an Eldritch Invocation option called Kiss of Mephistopheles:
Prerequisite: 5th level, the Fiend Patron, eldritch blast cantrip
You can channel the fires of Mephistopheles through your eldritch blast. When you hit a creature with that cantrip, you can cast fireball as a bonus action using a warlock spell slot. However, the spell must be centered on a creature you hit with eldritch blast.
The new UA allows the warlock to cast fireball as a bonus action — essentially building in the Sorlock in an invocation without requiring a multiclass into Sorcerer.
But the question is, do you actually need to know the fireball spell (as in, choose to have fireball take up one of your Spells Known) to be able to cast it this way?