You can do it without an add on.
In thread list the threads have a tick box for moderation selection.
in thread view the drop down at top right has the option to select for for moderation
Select the threads (must be more than one obviously) and a dropdown menu appears at the bottom of the page. Choose merge.
nb: if you have a list of threads via a search, you can see the moderation tick boxes if you click on enable moderation.