Use WriteValue, drop the "hex:" prefix and the commas in the data.

For example:
 Code:
$sKey="HKEY_CURRENT_USER\Software\Microsoft\Office\11.0\Common\MailSettings"
$sNUL=WriteValue($sKey,"ComposeFontSimple",
	"3c0000000f0000e800000040f00000004e0d"
	+"7374000000000020417269616c000000000000000000000000"
	+"000100000084ea13000100000001000000",
	"REG_BINARY")