Unless I am mistaken, aren't all but two empires in CK2+ titular? (for the record, the empire in question was the Spanish empire). I made it in the normal way - raised crown authority to max, met all the other requirements, and bumped up from late feudal to imperial. If I check the de jure box on the empire though, it says my primary kingdom title (Aquitaine) is de jure (note that I have de jure drift turned off).
Ah, I may have made it possible to have imperial if de jure drift is off. But that's kind of a nonsensical rule for Plus... Since you need drift for other empires to become de jure.