You can manually add items one by one or you can download a csv file with the required tables, enter the items via excel or w.e. and re import it back into your store.
Sorry if it sounds confusing, without the stuff in front of me its hazy and I don't have the ftp or admin info at home since the stores are for clients at an art studio I work for.
Edit: found another article for ya:
http://www.magentocommerce.com/wiki/...-a-new-product