Found a way;
The specific problem I faced was how to disable two fields, but let standard mapped data to be displayed in them.
To cater this requirement I used the following;
Include: MV45AFZZ
User Exit Name: USEREXIT_FIELD_MODIFICATION
Enhancement Name: -Any name you want-<br/>
I created an Enhancement and wrote the following code;
"Specify the condition
IF VBAK-VKORG = '1234' AND ( sy-TCODE = 'VA02' OR sy-TCODE = 'VA01' ) AND ( screen-name = 'VBAP-KDMAT' OR screen-name = 'VBAP-ARKTX' ).
screen-input = 0."disable input
MODIFY SCREEN.
DATA: i_tab_mara TYPE TABLE OF MARA WITH HEADER LINE.
DATA: l_maktx TYPE MAKT-MAKTX.
DATA: WA_MARA LIKE LINE OF i_tab_mara.
DATA: i_tab_vbap TYPE TABLE OF VBAP WITH HEADER LINE.
DATA: wa_vbap LIKE LINE OF i_tab_vbap.
IF sy-TCODE = 'VA01' .
SELECT SINGLE * from MARA INTO WA_MARA WHERE MATNR eq VBAP-MATNR.
SELECT MAKTX FROM MAKT INTO l_maktx WHERE MATNR eq VBAP-MATNR.
ENDSELECT.
VBAP-KDMAT = WA_MARA-KDMAT.
VBAP-ARKTX = l_maktx.
MODIFY SCREEN.
ELSEIF sy-TCODE = 'VA02' .
SELECT SINGLE * FROM VBAP INTO WA_VBAP WHERE VBELN eq VBAK-VBELN AND POSNR eq VBAP-POSNR.
IF WA_VBAP-ARKTX eq ''." Check if the fileds are empty, otherwise old data is overwritten
SELECT MAKTX FROM MAKT INTO l_maktx WHERE MATNR eq VBAP-MATNR.
ENDSELECT.
VBAP-ARKTX = l_maktx.
MODIFY SCREEN.
ENDIF.
IF WA_VBAP-KDMAT eq ''." Check if the fileds are empty, otherwise old data is overwritten
SELECT SINGLE * from MARA INTO WA_MARA WHERE MATNR eq VBAP-MATNR.
VBAP-KDMAT = WA_MARA-KDMAT.
MODIFY SCREEN.
ENDIF.
ENDIF.
ENDIF.