Yes it is.
Take the theme you want to use, extract it ( after renaming it to *.tar ), open the xml file in the archive and edit it like that:
<?xml version="1.0"?>
<!-- Created with Sony Ericsson Themes Creator -->
<Sony_Ericsson_theme version="4.5">
<Desktop_style Type="File" Source="grouped_desktop.swf"/>
<Author_firstname value="Human Interface Design"/>
<Author_lastname value="Creative Design Center"/>
<Author_organisation value="Sony Ericsson"/>
Just change grouped_desktop.swf to the filename of your flashmenu.

Save, pack your menu as tar again, rename to thm and upload to your phone. then it should work.