Alexedit, the balance point of your model will be the same when the extra wheel is fitted as it is now. So mark where it is. Add the support structure you need to move the mainwheels back so that the axle centre is just behind this point. The closer to the balance you get them, the easier it will be to rotate at takeoff. Too close and the model will still want to sit on its tail, 1/2" behind is enough. Fix the new nosewheel and associated bits and check the stance (slightly nose down) and balance point, move the battery around until the model balances as before. Now you can go fly again. The model will be heavier and have more drag, so it will be slower and less maneuverable, but, you can now taxi around really easily.
Evan, WB #12.