I've done it with my Balsa USA Stik and it's a common conversion on, for instance, a Sig Kadet Senior. 'Tis fun! You've gotta move the main gear forward, ensuring that there's enough strength in the fuselage at that point. It should go, as a starting point, probably as far ahead of the CG as it is behind the CG now.

Then you've gotta engineer a tailwheel. It can be fixed, swivelling, or steerable via the rudder. Fixed will work, but ground-handling suffers. (I owned a tail-skid plane for a while.....think about you're gonna turn the plane on the ground. I had to give it down elevator to get the tail up. Do you really wanna do that?) swivelling is easy to mount. Steerable will take a little head-scratching, but is easy.....just eyeball your tail and see which commercially available tailwheel will work for you.