Hi Deepa,
For SRM 5.0,please refer to the below note for adding custom fields under the BASIC DATA screen
Note 672960 - User-defined fields 2.pdf
To control display of custom fields based on the transactions,please implement BADI BBP_CUF_BADI_2,method MODIFY_SCREEN.
There is another badi which is used to control the display of the links DESCRIBE REQ,CATALOGS based on which you need to control the custom fields.This BADI is BBP_SC_MODIFY_UI.If you check tehe interface paarmeters of this BADI,there is a parameter IS_SCR_ITMCHOICE which actually holds the values for all these links and the values also can be seen under the data type BBPS_SC_ITMCHOICE_ACTIVE.
So in the BADI to control display of custom fields,you would need to check if simlart values for the links/Trnsaction code are populatd in some field or you could also check SY-TCODE to see if that suffices.