You have a good question. I haven't built the Mig 3 yet, but I have built over 150 FG planes, so I'll give it my best to help you.
Adding the dihedral won't be too difficult. You will want to split the TOP wing surface all the way to the front edge of the airfoil. When setting the angle of dihedral, bend the lower wing surface.
To connect the upper wing surface again, I recommend using a joiner strip. 20# typing paper will work well. Cut a strip of paper about one centimeter (1/2 inch) wide, and long enough that it just stops short of the back of the wing (don't want it to bunch up there).
I would probably glue the strip to the outer part of the wing first. That way, you could go in from the opening in the middle of the wing with tweezers or something similar to secure it to the "wing root". Stuffing the wing with tissue/kleenex usually helps keep the shape and strength of the wing intact.
I hope this helps. Be sure to post some pics of your Mig when you're done.