this is only possible via an indirect way. Please search the forum first (e.g. with google) for previous discussions and solutions on this subject
Is this in the jBPM roadmap and in Jira to make this easier?
http://jira.jboss.com/jira/browse/JBPM-613 overall meta issue (not all others below are related to this. I'll 'fix' that tonight)
http://jira.jboss.com/jira/browse/JBPM-679 (no type-safety, no 'fix-version')
http://jira.jboss.com/jira/browse/JBPM-70, not assigned to a version to be fixed
http://jira.jboss.com/jira/browse/JBPM-228, would allow what is requested but in a little to complex way (it is not wrong to have a little more flexibility anyway)
http://jira.jboss.com/jira/browse/JBPM-807 one seam related request
http://jira.jboss.com/jira/browse/JBPM-72 type declaration for variables, no 'fix version' assigned