You don't. You cannot add a new column to a table while it is in use.

Neither do you need to. If you use a relational design, you will have many
records in a table (probably in a related table) instead of many fields. It
is then dead easy to add related records instead of the unproductive and
unnormalised approach of adding new fields.

For more information on how to design related tables like that, read up on
"normalization". Here's a starting point:
   http://home.bendbroadband.com/conradsystems/accessjunkie/resources.html#Database
Design101