You can't do what you propose. Here's a quote from the setup instructions for a SpamCop email account:
QUOTE
Don't forward mail (or use POP) from an account at another ISP, then forward mail from SpamCop back to that same account. The other ISP will forward back to SpamCop in a mail loop. When SpamCop detects a mail loop, it will bounce the mail back to the original sender. You won't be able to receive any email at all if you have a mail loop set up.
Your best bet is to have the mail grabbed from Yahoo either *stay* in your SpamCop account (in which case you'd use SpamCop webmail or a POP or IMAP client on your computer or cell phone to access the messages) or have the messages eventually forwarded to yet another free account, perhaps on GMail or the like.
The downside is that you'd not be able to respond to people using the "From" of your original Yahoo account. While this is technically possible in SpamCop webmail, by using the "identities" function, it's highly unadviseable, in that many systems are specifically programmed to reject messages that appear to be Yahoo.com forgeries, which is what a message sent from SpamCop webmail using a Yahoo.com return address would be.
So, I'd suggest moving all that mail out of those folders to a new account....either at SpamCop or elsewhere, and notify people of your new address, either SpamCop.net or otherwise.
DT