Hi Igor,

You can upload a file using Nintex Forms and Forms attachements. If you want to do it during the workflow execution, you should create a flexitask for those users requesting them (in the form) to attach a document. It will be set as attachments, but you can post it as well inside a simple docLib (Copy attachment from list item to document library).

Then when you get it, the workflow can easily apply properties to your document, there are actions for that.

I hope it helps