Sharepoint Document Upload Page – Passing URL Variables

Throughout my SharePoint site, I have several document repositories that are tied to primary keys from an external database. I have added custom columns in the document library metadata fields so that we will know which SharePoint documents correspond with which table entries. As a requirement, we need to have document uploads that have these fields automatically populated. For instance, I'd like to have the following url:

./Upload.aspx?ClassID=2&SystemID=63

So that when you upload any documents to this library, it automatically adds the ClassID and SystemID values to the corresponding ClassID and SystemID columns outlined in the SharePoint document library fields.

Is there any quick or easy way to do this, or will I have to completely rewrite the Upload.aspx script from scratch?