Here's a quick hack that's nowhere near acceptable for the long term. All it does is:

(1) /hload the chanfolder section of mirc.ini into a hashtable
(2) create a new item for the active channel at the active network, with the setting for autojoin-on-connect being checkboxed
(3) /hsave the hashtable back to mirc.ini as a replacement chanfolder section

It doesn't check to see if you already have it there, so if you end up with duplicates you'll need to kill the extras. Even though the items are saved in random order, the Alt+J window always shows them in alphabetical order

To create this,
((AA)) save a backup copy of mirc.ini in case you mess things up. If it doesn't have a new timestamp you found the wrong one. It will be the one you see in this command: //echo -a $mircini
(A) press Alt+P to open the popups that control the right-click menu
(B) If it's not already there, use the 'view' dropdown menu for the popups editor to show the menu for things to do when you right-click in channel. If you've done nothing to it yet, the default popup should already have a line like: Channel Modes:/channel
(C) CopyPaste the new commands onto blank lines below the existing line(s)

Add 2 Favs:{
  hfree -w foobar
  hload -im foobar $qt($mircini) chanfolder
  hadd foobar new $+($chan,$chr(44),$chr(44),$chr(44),$qt($network),$chr(44),1)
  hsave -sin foobar $qt($mircini) chanfolder
  echo -a open alt+J window and see $chan @ $network added to your Fav List even if it already exists

(D) When you right-click in any channel, you'll now see a new menu item named "Add 2 Favs"