I have got the answer, we need to pass MIME TYPE 'APPLICATION/MSEXCEL; charset=utf-16le' and add 'FFFE' in starting of hexadecimal output that we get after processing function module SCMS_STRING_TO_XSTRING
code is as below:
CALL FUNCTION 'SCMS_STRING_TO_XSTRING'
EXPORTING
text = text
mimetype = lv_mimetype
* encoding = lv_encoding
IMPORTING
buffer = lv_report_xstr
EXCEPTIONS
failed = 1
OTHERS = 2.
IF sy-subrc = 0.
CONCATENATE cl_abap_char_utilities=>byte_order_mark_little
lv_report_xstr
INTO lv_report_xstr IN BYTE MODE.
ENDIF.