There is no "out-of-the-box" facility to do this, but if you are using Word
2002 or later you should be able to do it using VBA, using Word's Mailmerge
events. I cannot offer suitable code, but what you would need to do is trap
the "before record merge" event, then in the VBA code for the event:
a. determine which record you are processing
b. prompt for the data you want to insert
c. use ADO or DAO (say) to modify the appropriate record in the Access
database.

You might also need to ensure that when Word opens Access as a data source,
it does not open it for exclusive access.

Peter Jamieson