Accelerator only needed to be connected to the radio noise network because he had a very minor case of serious brain damage. He literally got shot in the head. Even when under the network he was actually weaker than he was before he was shot. The wings he has when in level 5.2 or whatever we want to call it are actually what are doing to calculations. Anyway, what is actually needed for a level 6 shift isn't fully known. Some things are theorized and some are confirmed.
One thing is possibly some connection to magic, this might just be an understanding though. Another is a change of how one sees the world, basically a form of enlightenment. Another is a change of how one sees themselves, different form of enlightenment. A sufficient level of power, Mikoto got this from the network, Accelerator could actually do this on his own before he got shot he just didn't get there yet. Finally, the thing Mikoto doesn't have but Accelerator does, a body strong enough or some way to make your body strong enough to handle the shift.
Mikoto, and Keiko here, might actually be able to do it if she was to master what Junko does. Junko is an eletromaster who physically enhances herself and has regen. Keiko is already doing that. I have no idea what Nozomi could do. Charlotte might be able to pull off what the endbringers do.
