I'm assuming these are empty...? Depending on weight, attaching something to the bucket could work pretty easily, but it will all be applying additional force to the bucket cylinder, pins, and linkages. You might be better off coming up with a way to attach your extension to the boom directly, instead.
How often do you dig with this machine? Could you take the bucket off completely and make up a mounting bracket with the same sort of ears? At the very least, then you'd not be toting the weight of the bucket around while you're trying to lift other things.
So to help us think about this, could you tell us if what you're trying to lift weighs more or less than a bucketfull of dirt?
Also, how long of an extension are you thinking about? 3 feet? 6? 10? The longer it gets, the longer the lever your load has to mess with you. Are you after more horizontal reach or more vertical height (to maybe fit more bulk hanging underneath)?
A few pictures of what you need to deal with might go a long way to help us understand and give useful feedback...