Hi Munu,
Using SAVE_TEXT Function Module you can upload Sales Text in MM Screen .
Check this Document "How to Upload Long Text into SAP Using Excel Sheet and SAVE_TEXT Function Module" this is for uploading PO text in SAP , Just Change Few things..
like Pass Text ID = '0001' for Sales Text(i.e. Long text) and Text Object = 'MVKE' .
Regard's
Smruti