Add button that will allow to add a set of fields to populate into same object?

How to create an add button to page that allows input another set of defined fields. for example, i have an incoming package which has 'X' items in it. I would like to assign the incoming package a unique ID while assigning each item its own unique ID to that package. This is quite easy if there is only on item, therefore is there a way to add a button that will allow me to add additional items?