To Add Tools to a Main Task

In the Customize dialog on the Tools tab, expand the Main Tasks tab.
Expand the User Tasks section and expand the open DGN library.
Select the Main task to which you want to add the tool.
Do one of the following:

Click and drag a tool from the open DGN file's toolboxes, a DGN library toolbox, or the Application Tools' toolboxes in the left pane to the selected Main task.

or
Click and drag a tool from a DGN library task or the Application Tasks in the right pane to the selected Main task.