Thanks Sathish for confirming that this can't be done at BO end.
I have implemented the below solution:
Create a single User EXIT variable (UE_TIME) for time Field. Create user entry formula variable (FV_NUMBER) of type Number - this can be used by BO/BW team only. one can use the Formula variable values to decide the time range for User Exit Variable when I_STEP = 2.
Example:
If FV_NUMBER = 1.
UE_TIME = 1st Day of Prev. Month - Last Day of Prev.
ELSEIF FV_NUMBER = 2.
UE_TIME = 1st Day of current month- Current Date.
ENDIF.
Thanks,
Sujal