You can do this at any time during or after entry of the requisition in Keystone Client.
Go to the Requisitioning menu -> Purchase Order Requests (RQ2110) screen -> Enter the Req #, Req Date, Vendor
Click on the 'Attach a new file' icon -> Select the document you want to attach to the requisition on the File Attachments (KC0146) screen.
Fill out the rest of the screen -> Click OK -> Finish entering the Purchase Order Requisition.
In KeyNet, you cannot do this until the end when you get the requisition number.