There are 3 options:

1. Filter the Subject tree to the desired set of tests in the test plan tree tab during the Select Tests process in the Requirements Module/ Test Coverage tab, using the filter icon. Then add the coverage from the Subject node.
2. Organize the tests to be associated with the single requirement into certain folders, and add the folder(s) to the coverage.
3. Use the QC API to programatically add the coverage.