I am now able to add custom menu items on click of "Change Type" icon. For adding custom shapes to the menu items, i followed this post.
Steps followed are as below:
1. Converted my custom icon to SVG format.
2. Converted SVG image to font file from fontello.
3. Copied the downloaded font files to bpmn-js assets directory.
4. Loading the new font style during application start up.
5. Made necessary changes in java script classes(ContextPadprovider, PaletteProvider.js)
Still, the custom image is not shown for the custom menu item. It is just showing a rectangle box(Please refer to "Custom Task 1" in the attached snapshot).
Please let me know if i am missing something.