(don't know if it is possible in soapUi free version)
What I'd recommend is to create a testCase called "id_generator" in a different testSuite, in my case called: "Blocks_loadTest_functions".
Everytime this testcase runs, a new id is generated and returned to a testCase property. (Id)
You can add a runTestCase testStep in your loadTestCase, set the property id as return value and set the runmode of the testStep to "SINGLETON_AND_WAIT" (First selectable option).
This will still be a (minimal) delay in your loadTest, since multiple threads cannot acces this testCase at the same time, but at least you can run multiple threads.
