When you purchase or try Xray for JIRA, often you already have one or more projects you're working on and want to test. Xray provides the ability to transform a regular project into an Xray Test Project without having to configure anything manually, even if you have custom schemes.
Enable Xray Test project through project Administration Page
In order to add Xray Issue types to your project you only need to click on Actions -> Add Xray Issue Types
Please bear in mind that reverting this action is not as straightforward. This is because Xray cannot know what configuration your project had before adding Xray issue types. Thus, you would have to configure your schemes manually to remove Xray Issue Types.