There are several "Day for Night" filters available from third party vendors. But if you want to use only the filters included with FCP I'd try a combination of the Tint filter and the 3-Way Color Corrector (for adjusting blacks and mid-levels).
Using the three-way color corrector to desaturate the video, darken it, and make it more blueish would probably be a good start. But to be honest, you're not going to get a 100% realistic look unless you actually shoot the video at night.