It is possible that your monitor will support the Wii's 480p resolution: it's probably 640x480@60Hz. However, the signal put out by the Wii is not a signal that your computer monitor will understand. Even the component output is in a different format. It's still putting out YPbPr, whereas your monitor wants RGB.

You can certainly get a converter, but the fact that you've got a 5BNC monitor has no bearing on that. Those 5 BNC cables are expecting the exact same signal that the monitor would receive over a normal HD15 VGA cable.