Dear Jyosh
Please try to use Additive cost through CK74 in your case for external services. Don't forget to tick additive cost button in your Costing Variant and Valualtion Variant, other wise it will not pickup additive cost. For your help below are the links on additive cost :
http://help.sap.com/saphelp_46c/helpdata/en/7e/cb81fa43a311d189ee0000e81ddfac/content.htm
http://help.sap.com/saphelp_470/helpdata/EN/7e/cb821443a311d189ee0000e81ddfac/content.htm
Probably it will solve your issue.
Regards
Rajneesh Saxena
Message was edited by: Rajneesh Saxena