It is possible to add buttons to the main menu for chapters, however it is not recommended, nor is standard practice to have the main movie and the chapter markers on the same menu page as they may cause issues with playback in a player.
The disc menu system will automatically create the Chapter menus with links from the main movie page as per the BD disc standard.
If you wish to try this adding buttons - in the disc menu Edit mode, click the + (plus) button to the right of the Design button option in the bottom of the right pane and link to the a chapter marker as required.
Depending on the content of the movie, eg you are creating a training video with different topics/sections, you may want to consider a multi-movie project - see this tutorial for how.