I don’t think you can do this entirely in Wappler. What you can do is create a PHP file which gets the dynamic expression and reference this as a PHP include file, where the expression would normally appear within the repeat.
I’ve done this with a dropdown menu which sets a cookie; this cookie is used to get the required expression. I couldn’t use Wappler for the include file*, so I used a standard MySQLi connection and query - obviously you could use PDO.
One other thing: if you change the expression, you’ll need to reload the page - so the PHP file can run on the server again to get the new expression. Eg you can add a static event ‘window.location.reload()’ on successful form submisstion (the form which sets the new session).
There may well be other/better ways to do this.
(I’m not sure why I couldn’t use Wappler for the second file - an ‘App Connect already running’ error message appeared. I expect it is possible and I was doing some wrong.)