I would use something like this

http://www.emailsmartz.com/asp/sender_pro.asp

Probably the easiest way to do it is to create a new column in your Excel
sheet that has a formula to concatenate the various bits of info you need,
then use that column when you merge to e-mail (assuming that's good enough
for your requirement.

e.g. if your firstname is in column A and your surname is in column B, a
formula such as =CONCATENATE(A2,".","B2","@thecompanyname.com")
might be enough - propagate the formula to every cell in the column.

If you don't want to disturb your existing sheet, copy it before adding the
column.

Alternatively, if you /just/ need a list of addresses and nothing else from
the spreadsheet,
a. create a "directory" type merge with the following fields and text:

{ MERGEFIELD firstname }.{ MERGEFIELD surname }@thecompanyname.com

Either insert the fields from the dropdown or use ctrl-F9 to enter the {}
then type the other text in the usual way.

Make sure there are two paragraph marks at the end oof the document.

Merge to a new document. Insert a column heading (e.g. "eaddress"). Save the
document, and use it as your data source.

Peter Jamieson