For CentOS 7, a custom menu items can be added via the Main-Menu application. Now I have upgraded to CentOS 8 this option is missing.
I have solved this issue by manually adding applications in /usr/share/applications/ directory. To add eclipse as a custom application add a new file /usr/share/applications/eclipse.desktop with the following contents in it:
[Desktop Entry]
Name=Eclipse
GenericName=Eclipse
Comment=Eclipse IDE for Enterprise Java Developers.
Exec=<path_to_eclipse>
Type=Application
Icon=<path_to_icon>
Categories=Application;