I got it:
Just place this in the mousewheel event of the container:

LPARAMETERS nDirection, nShift, nXCoord, nYCoord
IF thisform.ViewPortTop-ndirection>=0
	thisform.SetViewPort(thisform.ViewPortLeft,thisform.ViewPortTop-ndirection)
ELSE
	thisform.SetViewPort(thisform.ViewPortLeft,0)
endif

Thanks anyway...