There is an easy way which may not do everything you want and a harder way which can do everything.
1. The easy way: There should be a set of Excel Macros included with your release of QC for importing and exporting to the TD part of QC.
2. The hard way: you should be able to get the API for test cycles and write a vba macro to run the API.
3. The really hard way: Acquire enough access rights to walk the schema and write a odb routine in a vba macro to talk directly to the underlying database (probably MS SQL server on small enterprises, and Oracle on large ones). You want to update entries in the CYCLES table. (Not sure of the exact name because I am not currently working with TD/QC but have done so in the recent past.
4. You can, with some care, automate the TD interface using Mercury automation tools like WinRunner or QTP (and probably with other tools of an equivalent nature like silk). You could then use your spreadsheet as input to a datadriven WinRunner Script. WinRunner will not ordinarily record GUI map objects for TestDirector, so you may need to explicitly code these (The objects do show up in the spy).