Sign in as an administrator is ok, when I tried it, as long as he is also a moderator.
Look for the checkbox on the bottom left:
(If you do not have that, there is a problem for Xenforo.)
Place the cursor over it: "select for moderation"
Check it and you get a "Choose action" dialog box.
(Six moderator actions are available.)
Choose "Move posts"
Note that you place in the url of the thread you are moving to.
And when I just tried it, it placed the TO post as the TOP POST, and moved the existing post below.
And I think it renumbered the posts on the FROM.