How to link existing test cases to an existing test plan?

Hello,

We executed about 1500 test cases without really creating a test plan. So all the test cases(TER's) are orphans and are not tied to any test plans. Thus, we are not able to leverage any delivered reports..
So the question is - I have created a test plan and was attempting to associate one test case manually to the test plan, but was not able to find any place where I could do so.
1. How do I associate 1 test case to a test plan manually?
2. Is there an easier way to do this? Note that we have already loaded and executed the test scripts in the test case, so not sure if the excel utility can still be used or not.

Please let me know.

Thank you for all the help,
Yatish.