Groovy to add values to multiple REST steps

I have a testsuite of 60 testcases. They all contain on average 25 REST teststeps. In the Header tab of each request for them I am using a parameterised value - ${parallelSetup#userId} - to use a value stored in the properties of the testcase in the REST request. I would rather not have to manually go into each step and put this value into the Header properties. Is there a groovyscript that would update/add this to this section in the desired REST teststeps?