1、se11创建结构，必须包含行项目号POSNR字段，和其他需要扩展的字段，如记账码Posting Key、原因代码Reason Code
2、SE19实现BADI增强，这个增强是用来将BAPI_ACC_DOCUMENT_POST参数表EXTENSION2传入系统表
3、使用BAPI_ACC_DOCUMENT_POST参数表EXTENSION2，将扩展字段传入
详细步骤如下：
1、创建结构，se11，很简单，不再赘述，如下图：
2、SE19实现BADI增强ACC_DOCUMENT
通过ACC_DOCUMENT help文档知道，方法CHANGE用来完成字段的扩展，还有一个需要注意的是参考业务类型，这个一定要选对，不然执行BAPI的时候不会调用这个BADI，我用的是BKPFF，如下图：
激活这个BADI实现。
双击change方法创建，可以查看ACC_DOCUMENT的实现例子CL_EXM_IM_ACC_DOCUMENT（R/3 4.7版本，其他版本可能名称不一样），，将这个实例的change方法的代码直接copy过来，激活方法，代码如下
***********************************************************************
* Example to move fields from BAPI parameter EXTENSION2 to structure *
* ACCIT (accounting document line items). *
* The dictionary structure (content for EXTENSION2-STRUCTURE) must *
* contain field POSNR, (TYPE POSNR_ACC) to indentify the correct line *
* item of the internal table ACCIT. *
***********************************************************************
DATA: wa_extension TYPE bapiparex,
ext_value(960) TYPE c,
wa_accit TYPE accit,
l_ref TYPE REF TO data.
FIELD-SYMBOLS: <l_struc> TYPE ANY,
<l_field> TYPE ANY.
SORT c_extension2 BY structure.
LOOP AT c_extension2 INTO wa_extension.
AT NEW structure.
CREATE DATA l_ref TYPE (wa_extension-structure).
ASSIGN l_ref->* TO <l_struc>.
ENDAT.
CONCATENATE wa_extension-valuepart1 wa_extension-valuepart2
wa_extension-valuepart3 wa_extension-valuepart4
INTO ext_value.
MOVE ext_value TO <l_struc>.
ASSIGN COMPONENT 'POSNR' OF STRUCTURE <l_struc> TO <l_field>.
READ TABLE c_accit WITH KEY posnr = <l_field>
INTO wa_accit.
IF sy-subrc IS INITIAL.
MOVE-CORRESPONDING <l_struc> TO wa_accit.
MODIFY c_accit FROM wa_accit INDEX sy-tabix.
ENDIF.
ENDLOOP.
3、使用BAPI_ACC_DOCUMENT_POST参数表EXTENSION2，将扩展字段传入
**********************************************************************
*INTERNAL TABLE DECLARATION
**********************************************************************
*&—-G/L ACCOUNT ITEM
DATA: ACCOUNTGL TYPE STANDARD TABLE OF BAPIACGL09.
*&—CURRENCY ITEMS
DATA: CURRENCY_AMOUNT TYPE STANDARD TABLE OF BAPIACCR09.
*&—-RETURN PARAMETER
DATA: RETURN TYPE STANDARD TABLE OF BAPIRET2 WITH HEADER LINE.
*&—-it_extension2 ITEMS
DATA: IT_EXTENSION2 TYPE STANDARD TABLE OF BAPIPAREX WITH HEADER LINE.
*&—WORKAREA FOR ZEXTEN
DATA: WA_ZEXTEN LIKE ZEXTEN. ”ZEXTEN就是刚才SE11创建的那个结构
**********************************************************************
*赋值
**********************************************************************
*& EXTENSION2扩展字段增强部分
WA_ZEXTEN-POSNR = '0000000010'. "凭证行项目
WA_ZEXTEN-RSTGR = '171'. "凭证行项目原因代码
IT_EXTENSION2-STRUCTURE = 'ZEXTEN'.
IT_EXTENSION2-VALUEPART1 = WA_ZEXTEN.
APPEND IT_EXTENSION2.
*其他参数表的字段赋值如下例
HEADER-HEADER_TXT = 'TEST HEADER'."凭证抬头文本
HEADER-USERNAME = SY-UNAME. "用户名
HEADER-COMP_CODE = '1000'."公司代码
HEADER-DOC_DATE = '20090618'."凭证中的凭证日期
HEADER-PSTNG_DATE = '20090618'."凭证中的记帐日期
HEADER-DOC_TYPE = 'S1'."凭证类型
* HEADER-BUS_ACT = 'RFBU'."交易业务
WA_ACCOUNTGL-ITEMNO_ACC = '0000000010'. "会计凭证行项目编号
WA_ACCOUNTGL-GL_ACCOUNT = '1002010105'. "总分类帐帐目
* WA_ACCOUNTGL-ITEM_TEXT = .
WA_ACCOUNTGL-BUS_AREA = '8000'. "业务范围
APPEND WA_ACCOUNTGL TO ACCOUNTGL.
CLEAR WA_ACCOUNTGL.
WA_ACCOUNTGL-ITEMNO_ACC = '0000000020'. "会计凭证行项目编号
WA_ACCOUNTGL-GL_ACCOUNT = '1301040000'. "总分类帐帐目
WA_ACCOUNTGL-BUS_AREA = '8000'. "业务范围
*WA_ACCOUNTGL-ITEM_TEXT = .
*wa_accountgl-ACCT_TYPE = 'R'.
APPEND WA_ACCOUNTGL TO ACCOUNTGL.
CLEAR WA_ACCOUNTGL.
WA_CURRENCY_AMOUNT-ITEMNO_ACC = '0000000010'. "行项目编号
WA_CURRENCY_AMOUNT-AMT_DOCCUR = '500'. "金额
WA_CURRENCY_AMOUNT-CURRENCY = 'RMB'.
APPEND WA_CURRENCY_AMOUNT TO CURRENCY_AMOUNT.
CLEAR WA_CURRENCY_AMOUNT.
WA_CURRENCY_AMOUNT-ITEMNO_ACC = '0000000020'.
WA_CURRENCY_AMOUNT-AMT_DOCCUR = '-500'.
WA_CURRENCY_AMOUNT-CURRENCY = 'RMB'.
APPEND WA_CURRENCY_AMOUNT TO CURRENCY_AMOUNT.
CLEAR WA_CURRENCY_AMOUNT.
*执行BAPI
CALL FUNCTION 'BAPI_ACC_DOCUMENT_POST'
EXPORTING
DOCUMENTHEADER = HEADER
*CUSTOMERCPD =
*CONTRACTHEADER =
*IMPORTING
*OBJ_TYPE =
*OBJ_KEY =
*OBJ_SYS =
TABLES
ACCOUNTGL = ACCOUNTGL
*ACCOUNTRECEIVABLE =
*ACCOUNTPAYABLE =
*ACCOUNTTAX =
CURRENCYAMOUNT = CURRENCY_AMOUNT
*CRITERIA =
*VALUEFIELD =
*EXTENSION1 =
RETURN = RETURN
*PAYMENTCARD =
*CONTRACTITEM =
EXTENSION2 = IT_EXTENSION2
*REALESTATE =
.
IF RETURN-TYPE NA 'EA'.
CALL FUNCTION 'BAPI_TRANSACTION_COMMIT'
EXPORTING
WAIT = 'X'.
ENDIF.
如果还需要扩展其他字段，在结构ZEXTEN加入，然后在调用BAPI前对相应字段赋值，就行了。