You can assign any hotkey to any command. The easiest way is to edit \SPB_Data\pcbenv\env file and add a following lines:
alias ~1 'define grid ; FORM grid all_etch all_etch_x_grids 0.125 ; FORM grid all_etch all_etch_y_grids 0.125 ; FORM grid done'
alias ~2 'define grid ; FORM grid all_etch all_etch_x_grids 0.25 ; FORM grid all_etch all_etch_y_grids 0.25 ; FORM grid done'
There "Ctrl-1" will set etch grid to 0.125mm and "Ctrl-2" will set to 0.25mm.