SONY a77 MKii custom setting

[Zastavia]

I am setting up my custom modes on the dial on my a77 MKii and have got everything figured out, but was wondering if it's possible to change the shooting mode as it seems to be locked on program (P). I noticed when customising the fn menu you can set a box as 'shoot mode' but it's always locked out and says 'setting currently unavailable'...would be nice to set it as shutter priority if possible?