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.
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