How to play impot games in 60 Hz on PAL DC ???

I got a game shark wich allow me to choose to run games in NTSC or PAL, but when i try in NTSC, only the superior half of the screen is shown on the inferior half of my TV !!! ((((

What a shame to don't let this paramter be available in the bios of the DC as the language,...Etc
The choice in PAL game of 50 or 60 Hz is good thing but if it have been a parameter, one time you have decided your mode, you don't have to take care of this, to lose time when lauching game, and i'll have been able to play import in 60 Hz !!!

No possibility to add a switch like it is easy to do on a saturn (and megadrive ) ???

Thanks !