how to attach cost elements to the wbs elements

Hi gurus,

I have created project with wbs elements. my client is asking below requirement and they want's to use GL's for all projects.

whenever they are posting fi transactions they provide GL only then automatically wbs will update in transaction.whenever we give other GL for wbs elements it must through error message.

where i want to link this GL and wbs elements and what are the impacts for assignment.

so please suggest.....

         Wbs Element                                Cost element

       1. wbs purchase cost                   4000001to 400100

       2. wbs other charges                    400200 to 400300

       3 .wbs  services                           400400 to 400500.