Setting a reg key with variables? And calling with batch?

[-Gamah]

Here's what my script uses to find someone's game directory (this is an automatic addon installer)

Function .onInit

Call GetSteamAccountName
Pop $R0 ; CaDDy-
Pop $R1 ; E\Games\Steaaam

# Check if Steam is installed
StrCmp $R0 "" 0 +3
MessageBox MB_OK|MB_ICONSTOP $(SteamNotInstalled)
Abort

# It is installed so set INSTDIR
StrCpy $INSTDIR "$R1\SteamApps\$R0\garrysmod\garrysmod"

The "$R1\SteamApps\$R0\garrysmod\garrysmod is what i want written to a registry key.

This brings me to the second question, is it possible to call up a registry key as a variable in a batch file? If so, how would I do so?