Have you looked at poi?
Jakarta Poi I had to do something similar to what you're asking. Poi has an excel table represented as a high level class (you can modify cells, rows etc. easily without worrying about the low-level event model). All I did was extend the HSSFWorbook class (it represents a workbook). You could make the extended workbook take a jtable as an argument, populate the first row with the column headings (or whatever), and the other rows with the row data. Or extend the sheet model and add the sheets to a programtically created or read workbook.
Check it out, I've found Poi extrodinarily useful in several circumstances.
Michael Crutcher