BAPI_ACC_GL_POSTING_POST - Post to Customer Number

I am trying to work on the above BAPI in XI 3.0 and R/3 Enterprise.

We wish to pass a customer number instead off a GL Account for one line of the posting, but I get the following error:

Required field GL_ACCOUNT was not transferred in parameter ACCOUNTGL

If I use the Customer GL Account, I get this error:

Account 2840 in company code 1000 cannot be directly posted to

Is it possible to change the posting key so we can post to Customer number without a GL Account?

If you need any further information, do not hesitate to ask!