Extend Existing Block with Custom Block Toolbar Control https://jeffreycarandang.com/how-to-create-and-extend-block-toolbar-controls-on-wordpress-gutenberg-block-editor/

existing-block-controls.js

/**
 * Add Custom Block Control to Existing Block
 */

import Controls from './controls';

const { createHigherOrderComponent } = wp.compose;
const { Fragment } = wp.element;

const allowedBlocks = ['core/group']; // Enable control to existing Group block

/**
 * Add custom attribute
 */
function addAttributes(settings) {
	// Check if attributes exists and compare the block name
	if (typeof settings.attributes !== 'undefined' && allowedBlocks.includes(settings.name)) {
		settings.attributes = Object.assign(settings.attributes, {
			theme: {
				type: 'string',
				default: '',
			},
		});
	}

	return settings;
}
wp.hooks.addFilter('blocks.registerBlockType', 'example/add-atttibutes', addAttributes);

/**
 * Add Custom Block Controls
 */
const addBlockControls = createHigherOrderComponent((BlockEdit) => {
	return (props) => {
		const { name, isSelected } = props;

		if (!allowedBlocks.includes(name)) {
			return <BlockEdit {...props} />;
		}

		return (
			<Fragment>
				{isSelected && <Controls {...props} />}
				<BlockEdit {...props} />
			</Fragment>
		);
	};
}, 'addBlockControls');

wp.hooks.addFilter('editor.BlockEdit', 'example/add-block-controls', addBlockControls);