If you can modify your business process to use service masters AC03 or service materials of material type DIEN, then you can determine the account by enabling valuation class in the rules for WRX in OBYC and maintaining the freight clearing G/L against corresponding valuation classes.
If not, you will have to use the exit LMR1M002. For this, you will need to enable account modifier in WRX rules and maintain the G/L accounts. Then you can determine separate accounts using some ABAP in the exit.