You might be able to do this programmatically if you try this:
EnvironmentImpl environment = processEngine.openEnvironment();
try {
task.createVariable("variableName", "variableContent", null, false);
transitionChoiceVariable = task.getVariableObject("variableName");
transitionChoiceVariable.setExecution(null);
} finally {
environment.close();
}
this should create a variable that is no longer linked to the execution but still to the task