Hi Swathi,
If you have written any code in GET_XXX method for displaying the Custom fields, the values of the fields will not be downloaded to excel.
You can implement CRM_BADI_RF_Q1O_READ BAdi to overcome the above issue.
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"Code Snippet
CHECK iv_result_structure_name = 'CRMST_QUERY_R_SRV_REQ_BTIL'.
CLEAR et_result_structure.
CALL FUNCTION 'CRM_BSP_OIC_1O_READ_FROM_RF'
EXPORTING
it_object_key = it_object_key
iv_screen_structure_name = iv_result_structure_name
IMPORTING
et_screen_structure = et_result_structure.
By looping in to et_result_structure set the custom field values in .
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Regards
Rajarama U