Solution: 1) Go to the Videos tab on the channel you want all the videos from. 2) Scroll down and keep clicking Load More until all the videos are displayed on one page (easiest to do this on Grid setting). 3) Press Ctrl+Shift+K (in Firestorm, not sure about other browsers) to open the console. 4) Enter the following:

javascript:
function d(){
   var el = document.getElementsByClassName('yt-uix-button yt-uix-button-size-small yt-uix-button-default yt-uix-button-empty yt-uix-button-has-icon no-icon-markup addto-button addto-queue-button video-actions spf-nolink hide-until-delayloaded addto-tv-queue-button yt-uix-tooltip');
   if (el.length > 0) {
      el[el.length-1].click();
      setTimeout(d,500);
   }
}

5) And then enter:

d();

And it should add all the videos on the channel to your Watch Later list. You will have to wait for it to "click" on each button (half a second per video) so it may take a small amount of time depending on how many videos there are.

6) Repeat steps 1 - 5 for each channel you want all the videos from. 7) Go to your Watch Later playlist. 8) Click the the vertical squares near the top left of playlist and choose Add all to... 9) Pick a playlist to add them to or create a new one.

Done. You now have a playlist containing all the videos from multiple channels.