In the details window of the specific test suite, click the menu icon on the right side of the title row and select Add Test Cases. Select the required test cases from the popup and click Add. (If you are adding the first test cases to a test suite, there will be an introductory screen with an Add Test Cases button.)
Note that in a build, you will be able to add requirements, test suites, and standalone test cases. All test cases linked to a requirement will be included in the build.
Users will need view and edit permissions in the test coverage module to add test cases to a test suite.