you can propagate data from one table to another if you have established relationships and employ cascading on the child field. I wouldn't call that "auto populate". It can also be done via form design if the form and underlying query is designed correctly and the query is updatable.
EDIT - I'm not disregarding any advice about what should go into some other table. I tend to interpret the first post as being about adding the PK field (autonumber or not) to the related table, not duplicating data.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.