Can I put a front tire on the rear?
No. Don't do it. It's a bad idea, and there are plenty of good tire options for the EX250. It's completely unnecessary.
This question comes up when someone wants to get a particular tire, but it's only made as a front in, say, 130/90-16. It's still the same size, so it sounds OK - but it's not. A front motorcycle tire is designed specifically for the front.
Tires are directional for a reason. That reason is for the dispersal of energy. A front tire works on energy from braking and the rear on energy from accelerating. Tires are "wrapped" (for lack of a better term) and can come apart if pushed in the wrong direction. It's like trying to find the end of a roll of tape: Run your fingernail one direction and it just pushes the end into the roll and slips off the end. Run it the other way and your fingernail catches the tape and pulls it up and away. So, to run a front tire on the rear and not run into a delamination issue, you need to flip it around backwards. But, by doing that, you 100% negate all the water dispersing properties of the tire.
Fronts also have a rounder profile and shallower tread, so they won't last as long and won't handle as well as one designed for the rear.
And to anticipate the next question: Putting a rear on the front is even worse.
It all works out to be a question of why add more risk to the already inherently risky activity of motorcycling when there are very good tires in the correct sizes and orientation for your bike.