this issue has been raised on their github page and later added to their documents.

below is the copy&paste from their doc,
registering icons

import { registericons } from '@uifabric/styling';
import icon from '@fortawesome/react-fontawesome';    
  icons: {
    'home': <icon icon={['fal', 'home']} />,
    'homesolid': <icon icon={['fas', 'home']} />,

using it

import { button } from 'office-ui-fabric-react/lib/button';

const icontest = () => <button iconprops={{ iconname: 'homesolid' }} />

Related Query

More Query from same tag