New record from a join query

I have 2 tables joined by a query. The first table is a fixed list of
types and contains 2 fields: Type which is a number and Description.
This table lives in the front end db. The other table contains user
descriptions and also has 2 fields: Type and UserDescription. This
table lives in the back end database.

The query shows all records in the front end table plus records where
"Type" is equal. So basically the user can keep the default description
or enter their own.

I have a form based on this query.

If a matching record already exists in the backend table, then I can
change the UserDescription ok, but if no record exists, I can't create
a new record by entering a description into the blank field. Does
anyone know a way I can do this?

The only thing I can come up with, which is really ugly, is to trap the
BeforeUpdate on the form, work out if a record already exists and then
create or update the record by running a query, then cancelling the
BeforeUpdate and refreshing the form.

Any ideas appreciated.

Andrew