Are they all @thesame.com?

If your data is in one column, I'd insert a new column and use a formula:

=a1&"@emailaddress.com

Copy|paste special|values and toss the original column.

And if you save the file as a .csv (comma separated values), then you can
specify that inside Outlook for importing.

(I might even export the existing contacts as a .csv to see what it looks like
so I had something to match.)