Dear Zain,
You can attach external document directly to PO or PR using menu path System > Services for Object. > Create Attachment.
Alternatively you can first create DMS document in CV01N attach document to it and the link DMS document to PO or PR item.
If you try this and you still can't create an attachment - let me know what system messages you get or provide any screenshots.
Regards, Maria
PS: You should better place this question in MM forum.