yes you can, I know I have with my EZ2 atleast (depends on if the ez4 fits into the gba, cause if u have a smaller ez4 as I have it's not possible).. you could say pogoshell is a frontend, a frontend is somewhat of an os, but in pogoshells case it's like a menu for selecting games (selecting i.e. nes roms and they are played via a plugin), playing movies, playing music which you can stylize yourself. I remember making a Castlevania: Harmony of Dissonance skin which totally rocked
I had nes, snes, gg, mastersystem, pc-engine, gb plugins for pogoshell
and I also had some music plugins such as midi, nsf (nes sound format), spc, xm, mod, s3m.
I also had some image plugins to view manga and similar.