Hi Kim,
There are several threads on this topic which you can read here:
https://forums.adobe.com/thread/447115?q=add%20attachment
https://forums.adobe.com/thread/543979?q=add%20attachment
NOTE: Although it is possible to add this feature to your form, you will run into issues with those individuals who are using free Adobe Reader to complete the form (potentially the majority of your form users). This is because the form requires special rights (aka Reader Extensions) in order to grant Reader users the ability to attach files using the free application.
The threads above talks about this hindrance, but I wanted to give you the heads up so you don't spend too much more of your time trying to get it to work.
Now, if you can guarantee everyone who will use the form will be using Adobe Acrobat then you can certainly continue to pursue the effort. Or, you can try to find someone who can add the required Reader Extensions to your completed file which requires special software (i.e. these are NOT the same reader rights that can be applied using Acrobat). The problem is...each time your form changes, you will need someone to reapply the rights to your document. And in case you're wondering about doing it yourself, the extension rights are very expensive.
Perhaps a much easier approach would be to simply have them attach the additional documents to an email.
Hope this helps.