The way I can see that you can do this (using v9) is with a calculated field.
I came up with this formula which forces two digit day, month, and year:
[SIZE=2]right("0" + trim(str(month(mydate))),2) + "/" +/SIZE
right("0" + trim(str(day(mydate))),2) + "/" +/SIZE
Edit: This is a Character field type.
2nd Edit: I missed the part about exporting the dates to Excel. Sorry. Why not just format the two digit year in Excel using cell formatting? You'll need to do that anyway, because when you export mm/dd/yy to Excel, Excel converts it to mm/dd/yyyy anyway, no matter how you finesse the format with Monarch.