You can (generally) speaking use JSTL's set in UIX JSP pages, and (mostly) use JSTL JSP tags with UIX JSP tags.
So, you should just be able to use <c:set>.
The major complexity in integrating the two is remembering that JSTL sets and iterates immediately, but UIX JSP evaluates EL expressions only once the last UIX JSP tag completes. This doesn't cause much of a problem with <c:set> or <c:if>, but does cause some major problems with <c:forEach>.
(A very similar problem is encountered when using JSTL tags with Java Server Faces JSP tags, so this is just a painful fact-of-life.)