How can I export a single wav with multiple independent tracks?

Create a new wav file in Waveform view with the number of channels you want. So if you have four stereo channels, then you'll need eight channels in your new file. Now open all of the tracks you want to include in this file. On the right hand side of the screen with the multiple tracks, you'll notice that the tracks are numbered. If you click on the numbers you can temporarily deselect them. If, for instance one of your files was stereo, then turn off all except two of the channels. Now if you go back to your imported file and copy it, you can click back onto the multichannel file and paste it. It will now take up two of your tracks. Obviously if you've only got mono tracks, then just leave a single one active. When you've done that, you select the next track to fill, and make sure that it's the only active track. Go back to the files you want to insert and select the next one and copy and paste that - same procedure. When you're done, save the resulting file and you're done.

It's almost easier to do than it is to describe, but to make the filling process as clear as I can, here's a screen grab of two filled tracks, plus the next ones just pasted: