There is no way that can be done by fiddling with the field code.  You could
execute the merge to a catalog (or directory in Word XP and later) mail
merge in which you arranged the fields so that they produced the names in
the format of Mr John Smith and then you could use a Wildcard Edit>Replace
to dispense with the unwanted part of the forename.  Then that modified file
could be used as the data source.

Without using VBA and Word's Mailmerge events, there are a some approaches
you can take to this (other than storing the initial separately in Outlook),
but they are all awkward to use. e.g.

1. If you have Access, you can link a table to your Contacts list in
Outlook, then write an Access query that takes the leftmost character of the
first name, then use that as the data source. The problem with that is that
Access only retrieves some of the fields from the Contacts folder and they
may not include the ones you need. You may also find that Unicode characters
do do transfer properly.

(To do this, create a new Access database, then use File|Get External
Data|Link Tables...then in the Files of type... dropdown at the bottom of
the Link dialog box, select Outlook() and follow the dialogs. I leave you to
work out how to do the query and so on).

2. Use 26 IF fields in your Word document as follows:

{ IF { MERGEFIELD First_Name \*Upper } = "A*" "A" ""
}{ IF { MERGEFIELD First_Name \*Upper } = "B*" "B" ""
}
and so on down to Z (assuming you only need worry about the 26 basic Latin
letters

Peter Jamieson