My answer: users cannot do this themselves. A developer must do this, copy for example Server Objects about which everybody agrees that they are worthwhile into the QVW base document, and make them fixed and visisble to all.
If you mean: can users create their own visualisations and "add" them to a published document, then the answers is Yes!. Have a look into Server Collaboration. These visualisations will not really be added to the document but kept on the server and combined with the objects in the document the moment it is opened in the AccessPoint.
As Perter mentioned, there are a couple of options.
We normally allow user collaboration on all documents. Users create objects, sheets, bookmarks and reports for their use, or share will limited or all other users.
If a set of user created objects are wanted as core components, users can request these objects get copied to the document by a developer. They will be validated and processed through the change management procedure.
We have found this provides optimal flexibility, while maintaining tested and stable functionality.
I hope this helps,