It depends on how your xlsx file looks. If it is close to SBTab format you could do some basic conversions to your XLSX and try to create the SBML with SBTab.
If you know a bit of python I would recommend to write a simple script to create the SBML from the excel file.
I am updating the fbc information this week, so there should be better documentation on how to easily create an SBML model with fbc information.
If you have any questions let me know.
Best Matthias