Using the Helix Importer createBlock function

import.js

/* global WebImporter */

/* Create a block of name/value pairs (eg. Metadata block) */
const metadata = { 
  ...WebImporter.Blocks.getMetadata(document),
  keywords: 'keywords',
  "Publication Date": dateFromSourceDom,
};

const block = WebImporter.Blocks.createBlock(document, {
  name: 'metadata',
  cells: metadata
});

/* Create a block from an array of elements */

// Single row block with three columns
const items = [
  [document.querySelector(''), 'a string value', [...document.querySelectorAll('div.someclass p')]]
];

const block = WebImporter.Blocks.createBlock(document, {
  name: 'custom block name',
  cells: items
});

// Single row block with variant
const items = [
  [document.querySelector(''), 'a string value', [...document.querySelectorAll('div.someclass p')]]
];

const block = WebImporter.Blocks.createBlock(document, {
  name: 'custom block name',
  variants: ['stretch-start'],
  cells: items
});