As I don't have any experience with Quicken, I can't tell you how to do this
BUT allow me to suggest an approach:
Open Quicken, and examine both the Open and Import commands (if there is an
Import command). What file types can be opened or imported in quicken?
Compare these file types to those that Excel can Save As. Are there any in
common? Perhaps Plain Text?
Another suggestion, If you simply change the file extension from .xls to
..qif, what happens?