Does anyone know how i can add the gas in the coilovers? to make it harder/stiffer

 

to be honest I dont think you can. If you want to make it stiffer there are adjustments on the top of the shock if you already set that to the stiffest point already and its not stiff enough get the Blistein PSS coil overs made for the GS430

 

As it needs to be regulated /measured, I don't think it is as easy as just adding it in and hoping for the best otherwise they would have different pressures.
The shop locally here has a conversion chart for the Advox. You may want to try to call them.

18062 Redondo Circle
Huntington Beach, CA 92648-1326
Phone: (714) 698-8300
Fax: (714) 698-8301

www.xs-engineering.com