You don't need to convert to binary - if fact it's not very helpful.

Here is one way to do it, the example shows how to set the bits for drive letters M,N and O. The bitwise OR (|) ensures that the bit is set - you cannot set the bit more than once if you use bitwise operators.
Code:
$dNoDrives=ReadValue(KEY,VALUE)
$dNoDrives=$dNoDrives | udfDriveToDWORD("MNO")
$=WriteValue(KEY,VALUE,$dNoDrives)
 
Function udfDriveToDWORD($sDrives)
	$udfDriveToDWORD=0
	While $sDrives
		$udfDriveToDWORD=$udfDriveToDWORD | Exp(2,ASC(UCase($sDrives))-ASC("A"))
		$sDrives=SubStr($sDrives,2)
	Loop
EndFunction
 
Function Exp($x,$y)
	$Exp=1
	While $y>0 $Exp=$Exp*$x $y=$y-1 Loop
EndFunction