Hi KarimD,
No, it cannot be done. You have to use the one that is provided Out of the box "OBS Unit" and take the help of Filter Mode attribute to select the option. Ideally you should not be needing another one filter field like that as you get the options to select all the hierarchies of OBS units from that filter field only. So I can confirm you that at as per the current design, it is not possible
Hope you question is answered. Have a great day ahead.
Thanks,
Abhisek Dhar