Zhen,
You can customize the Ribbon bar by modifying/creating DGNLIB file. What functionality\convenience is lost to the customer?
What Jan is saying is that you can also modify the ribbon programmatically if you are able to decipher the .net API.
If certain functionality\tools are missing from MicroStation, then add it in your self via MDL\.NET and link to it in the ribbon by adding it via the DGNLIB