Is there an aftermarket top over bag that can fit on top of OEM pannier ? Thank you
Not really. No footman loops on the OEM pannier to secure a bag with. That's the difference, the BMW panniers and Touratech Zega series, along with many others have footman loops built into them to secure things to the top of the panniers. Yamaha didn't do that. You can add your own, as WJB said above. Then use just about anything you want, including the bags sold for the BMWs or TT, KTM, Ducati, etc.
I use Rok Straps to secure things to the top of my Zega Pro panniers on my Super Ten.
Take a moment to add your location to your profile. It gives context to your posts and helps others better answer any questions you have.