Ok you aren't "posting Material document with movement type 201" but have already "posted Material document with movement type 201", right ? remember that the basic rule for material movement is:
The Document Concept, you can no longer change a document once it has been posted. If you made a typing error when entering a goods movement (for example, when you entered the quantity), you must reverse the movement with a so-called reversal document (see section Cancelling a Material Document).
Why Change a Material Document?
Although you cannot change a posted document, you can use the function "Change Material Document" to
- enter additional information for the document.
But SAP only provided MB02, and AFAIK there is no extension there. If your fields are (SAP point of view) only external ZZ_information, you could use MB_CHANGE_DOCUMENT or even Open SQL statement in this case (look at source of MB_CHANGE_DOCUMENT...) But if you are using some tools as logistic statistics you should also consider calling MCB_STATISTICS_LIS or even trigger MB_DOCUMENT_BADI_SAP / MB_DOCUMENT_BADI too if your fields have any impact there. Try to execute MB02 to change some value executing an Abap trace (SAT/SE30) and analyze list of called FM.
Regards,
Raymond