Mailmerge is really designed for inserting many instances of the same data
into a single document to create multiple instances of that document.  What
you want to do is in one instance of some data into many documents which is
quite different.  So mailmerge is not the thing to use.  At least not the
best thing.

The code in the item to which I pointed you can however be modified to do
what you want.  As this would appear to be required for commerical purposes,
if you do not have the knowledge to do that, then you may have to consider
paying someone to do it for you.

Though I agree with Doug in broad terms, if you already have mailmerge
documents set up it's more a question of
a. creating the data source you need
b. connecting the merge documents to the single data record you need and
performing the merge
b. automating the complete set of merges - you might for example either
process each document in a a particular folder or tree of folders, or
maintain a separate list and process that somehow.
c. paying some attention to saving your output documents, if you need to
save precisely what you produced (you need to do this whatever approach you
use if you are relying on machine-readable versions of documents for any
sort of legal purpose)

The problem with (a) is that if you already have merge main documents set up
to get their data from particular data sources, unless you make copies of
these documents and permanently each copy to the data source you need, each
time you use the mail merge main documents you will have to ensure that they
are connected to the correct data source for the purpose.

Peter Jamieson