Can't get WI attributes treated as variables in work item template

We have an issue where we have a customised attribute which is very similar to "Filed Against". When we create a work item template from this workitem type we are not given the option to check the box and treat this attribute as a variable. This means that if we export and import the template into a new project area and the Filed Against does not match, we cannot create new work items from the template. It looks like there are only two variables provided: filedAgainst and plannedFor.

<rtc_cm:filedAgainst rdf:resource="${filedAgainst:Category}"/>
<rtc_cm:plannedFor rdf:resource="${plannedFor:Iteration}"/>

Is there a way to edit the xml in the exported work item template to make other attributes treated as variables too? Or is this a RFE?