Drop down list in form

I have a form which is related to several tables.  One table is a 
single column list of data which I want to use for consistency.  I 
have a field where I would like to select from that table or add to 
it in a form.

An equivalent situation would be a city field in an address database. 
When adding a new customer it would be useful to select from a list 
of cities but also be able to add an new city without going out of 
the form to update the cities table.

I have tried adding a combo box and a table control to the form which 
seems to let me add to (what would be) the "cities" table but I can't 
seem to also get that new city into the current record at the same 
time.

I'm sure there's an elementary solution but databases are new 
territory for me.

Thanks for any suggestiuons

gary