It's not possible to create a custom toolbox for Bentley Map by adding its tools from the standard MicroStation customize dialog. This is due to Bentley Map's XML technology, which is different than MicroStation code.
You can create a custom toolbox using the MicroStation Customize dialog by entering the key-ins for the commands that you use. Review the document from the link at the bottom of this article to learn how you can perform these tasks.
Creating a Custom Toolbox in Bentley Map V8i