Create test items during runtime

I would say, up front, not exactly as you have described.  TestComplete does not support the addition of test items (script files, keyword tests, etc) during runtime AFAIK.

Since you are already using XLS files, my suggestion to you would be to build some sort of framework using DDT objects in TestComplete that read the XLS files dynamically and execute the tests as indicated.  This is a bit more of an advanced technique and takes some code work to build the structures but the ROI is good.  Once you have the basic framework constructed, all you need to do to add a new test case is to add rows to your XLS files.

So, your psuedo code would be something like:

Read XLS file(s)
Build ODT objects or arrays for each test case found in the file(s)
Execute a code routine to "step" through the list of objects/records
Output results to XLS file.

There is a set of topics that will help you down this path at http://support.smartbear.com/viewarticle/56219/.

If you're interested in something more advanced, I wrote http://blog.smartbear.com/software-quality/automation-framework-a-new-table-driven-technique-using-testcomplete a few years back.  I've developed the framework a little bit past that but I haven't had time to dedicate to making it more application agnostic yet.  As it is, this might be able to do what you want.  I'd be happy to help further in this.