JSTL sets its vars in the pageContext.  For example:

pageContext.setAttribute("foo", bar);

Therefore to get a JSTL variable use the pageContext within a tag or a JSP page.  For example:

// get item from pageContext and put in request
atg.servlet.DynamoHttpServletRequest drequest = atg.servlet.ServletUtil.getDynamoRequest(request);
drequest.setParameter("foo", pageContext.getAttribute("foo"));

Note that getAttribute assumes the variable is in the page scope.  If you want to get a variable from for example the request scope you would do this.

pageContext.getAttribute("foo", javax.servlet.PageContext.REQUEST_SCOPE);

For further reading please see PageContext.