Team, We need to add some User tasks as Tempo Actions for mock-up scr

This is not possible

If you're looking to mock up just the tasks, and your running version 7.5 or greater, the interface designer is a good tool. If you are looking to mockup the "Action" itself under the Actions tab, the process model will need to be created, but nothing needs to be in it. You just need something to reference in the "Application" when defining an Action.