A friendly representative of a company who helps small businesses sell products asked: "We're having problems forwarding mail from our customers back to our users due to the new Yahoo and AOL restrictive DMARC policy. If we add a DMARC record for our own domain name, would that help address the Yahoo/AOL bouncing issue? Would that explain to the ISPs that we're not spoofing when we forward on that mail?"
No, this wouldn't fix your issue. It's probably not a bad idea for you to implement a DMARC record for your domain, especially if the domain is one you use for email marketing or online retail and want to make it harder for bad guys to spoof it. (But be sure you learn more about DMARC before proceeding; I would recommend partnering with somebody like Return Path or Agari to use their tools and benefit from their expertise with regard to anti-phishing/spoofing and DMARC.)
The reason this wouldn't fix your issue is because the Yahoo and AOL DMARC policies affect only mail that has a Yahoo or AOL domain in the from address. Also, they have the potential to affect all/any mail with a Yahoo or AOL domain in the from address. What other domain you might have in the message or message headers has no bearing on that fact. Whatever DMARC policy setting you publish wouldn't override whatever policy setting the owner of those domains may have published. In other words, if it's AOL.com in the from address, it's always going to be the AOL.com policy that applies, no matter what.
The real fix for the issue is to figure out how to get it so only your own domain name shows up in the from address. That might necessitate a change in your message flow process. It might make you have to reconsider whether or not you forward on messages through your system at all. Or you might have to rewrite headers, if you still want to be able to forward on that mail.