Is there a way to automatically mail merge into a new document or append an
already created document?

Example ... I create an account update for client 12345 (the client number
'12345' would be a merge field). The account update also contains many merge
fields to give them information. Instead of printing out a hard copy of the
account update, I am wanting to have it just save it to a file called
accountnumber.doc (i.e. 12345.doc). If there is already a file created for
the account number, then it would just append the new information into the
file.

Is this possible?