There's no automatic way to pause a test. But during the runtime, you can reduce the number of virtual users that are running by decreasing the load.

If you stop all the virtual users from all the populations, the test will still be in progress but without any load.

Then you just need to add the virtual users again from your populations.

So it's not exactly a pause but at least you do not have to restart the test.

You can vote for that feature request here:
http://ideas.neotys.com/feedbacks/78856-pause-resume-the-...