Assuming that the Excel spreadsheet columns correspond to the required columns in LINKAGE format,
When in Excel, save the spreadsheet (using Save...Files) as a text file (tab delimited). You should give the text file version a different file name, so that you don't overwrite your original file. Do not save as a Microsoft Excel Workbook File! The resulting file will then be in ASCII, and can be attached in an email message, included in an email message, or directly used by the LINKAGE utility programs.
Thanks to Dr. Douglas Wilkin of NHGRI.