How to add attachments/uploads to a different record?

How to add attachments/uploads to a different record?

We created a multi-page application that updates records in a 'master' record. Each form collects different information and adds it to the master record by passing an ID between the forms. One of those forms allows users to upload documents. 

How can we create a form that allows the user to upload attachments and add those attachments to the 'master' record by ID? Is this possible?

Thanks,
John