Without knowing more about what you are trying to do with the AirPort Express base station, it would be difficult to give you an exact answer.
All we know is that you are asking can the AirPort Express be used to "boost" a Wi-Fi signal. The answer is "no." That's because when used for this purpose, the Express can only extend the Wi-Fi signal at the same bandwidth that it receives the signal. If you mean can it make the signal stronger, the answer could be "yes."