What do you mean? Like pressing Fn+other key or Caps+other key simultaneously? If it's that, the Epix already allows this. The only exception is that the Fn+other keys are reserved while in the Today screen for application shortcuts, but outside the Today screen, Fn+other key will still enter the appropriate symbol.
If you are looking for something different, then I'd be interested since it may be something I haven't seen on any Pocket PC to this date.