Prompting the sales team to upload a document at a specific opportunity stage

Hi!

I'm new to Dynamics CRM and was wondering if (or how) I can ensure our sales team upload a document at a certain stage of the sales opportunity process. I have it set up like so:

Lead --> Opportunity --> Scope Proposal --> Project Proposal --> Approval.

Under each stage I have various steps, and under "Scope Proposal" one of the steps is titled "Scope Proposal Sent". This is a simple yes or no selection, and upon the user selecting "yes", I was wondering if I could create a business rule + workflow that forces the user to upload the document right then and there. SharePoint is already integrated, and we currently require the users to go to the "documents" common tab under the opportunity and upload there. Has anyone done something similar to this? Thanks in advance!