In that case, you should create an Excel file with a list of names in once column and credentials in another. Then, when you get a new list from the client you can either:
a) use a lookup formula to retrieve the credentials for each entry and add them to the new file before merging; or
b) use a DATABASE field in the mailmerge to query the credentials workbook for the credentials that go with each name.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]