Pretty sure, the closest you can get is overriding the Quest button.
Xmlspawner has an override in PacketHandlerOverrides.cs for the Quest button, but that is *replacing* an existing button...
Otherwise, HR and Golfin are correct, in that it is all client-side and would need a custom client, like Orion or something, in order to change a lot of the gumps.
That being said; you can design a new paperdoll, using the same buttons and layout, but you can also add your own content. You just have to add it to the *graphic* of the paperdoll and then code it on the 'shard. If you go that route, there is not a *ton* of room to add stuff, so it would have to be something *small-ish*. 🙂