Can you use drop handlebar brake levers for bullhorn handlebars?

No way to tell.

Drop bar levers are meant to sit on the "outside" of a fairly sharp turn. Anything less, and the levers tend to bottom out against the bar before you've gotten full braking power.

If your bullhorn is a flip & cut drop bar, then you probably have enough curve to make it work.

But if it's a dedicated bullhorn, or (tri) base bar, then you may find yourself bottoming out.