You can extend a report in the Ajax browser to create new charts and expressions. However, the initial application/document/report will be created using the desktop tool first. Each user will have their own overlay with their modifications on top of the original application/document.
Users cannot add their own dimensions to the data model, but can use dimensions defined within the data model.
If the changes or tailoring were desired for everyone to use, a developer would incorporate/replicate the changes manually within the design/developer/desktop tool and deploy the changes to business users.
Hope this helps?