Hello Marcin,
Not only fill Extension table but you should also write some code on ABAP side. This process has two difference side. First you should fill data to Extension and secondly in abap side, you should catch the extension values and MODIFY some internal tables while the posting document is creating.
Shortly, it is not only enough to set some values in Extensions.
Here is the document about BAPI_ACC_DOCUMENT_POST Extension usage;
It is quietly clear to understand. However, if you do not develop something on SAP ABAP. It will also be hard to work for you.
