If you mean so all the layers turn...I don't think it'd be possible to just add layers onto the outside to make a 5x5x5. Basically the center, centermost edge, and corner cubies are held together internally like a 3x3x3 (the mefferts and eastsheen ones at least). The second and fourth layers are sandwiched in between the center and outer layers. A 7x7x7 ... I don't see how it would be possible, but I think you *might* be able to "split" the second-to-outermost layers of a 5x5x5 to make it two separate (and half-size) layers and get a fully-working 7x7x7 although you'd have to completely rebuild those pieces and probably everything else. How--I'm not sure--I've never tried to work it out. I know this wouldn't work with a mefferts cube because the the outermost layer "underlaps" the second layer a lot.
One thing that might be more easily possible is to add an additional turning layer on to each side that doesn't have the edge and corner pieces. Like this
or better yet this
! :-) only with functioning outer layers. You know how the outer layers of a Square-1 work? I bet something like that would work here.
And me wantses one :)