Data driven testing is one of the features that distinguishes SoapUI Pro from the the Open Source version. So don't expect any confortable support of data driven testing in the Open Source version.
The only work-around I could think of was to invoke the test case from a script (Groovy or shell script). This scripts handles the interation over some data source and transfers the data to the test case in a properties file. The name of this property file is specified via the system property soapui.properties.