Enhancing Function Module IDOC_INPUT_QUOTATION_CREATEFRO

Hi Guys,

              I have a requirement to enhance the FM IDOC_INPUT_QUOTATION_CREATEFRO.

But I am getting the following message..

Object FUGR 2031 cannot be enhanced; software component SAP_APPL cannot be enhanced

Exception of class CX_ENH_IS_NOT_ENHANCEABLE

If I wanna make changes some fields of the IDOC before it posted, how it can be done..?!

Thanks

Sam