Perhaps you need expand - - http://wiki.freeswitch.org/wiki/Mod_commands#expand

Example:

 expand originate sofia/internal/1001%${domain} 9999

In this example the value of ${domain} is expanded. If the domain were, for example, "192.168.1.1" then this command would be executed:

 originate sofia/internal/1001%192.168.1.1 9999

[edit]

-Avi