You should just be able to drag the .lmlm of Waluigi Pinball into the Multiplayer executable. Doing that should allow you to use the map in Multiplayer! Note that the map doesn't have Phonebooths so you'll have to use the Shift+F4 combo to bring up the Phonebooth and call a vehicle to you.
I went ahead and deleted the separate topic you made for this since this thread is still fairly active and it was asked here first. Please try to keep messages like that to a minimum and avoid making extra topics where possible. Thank you!