See the article here, explaining the technique I've used to do this.
Expand specific CMS items so that they span multiple columns or rows in you CSS grid.
DEMO
This CMS Collection List is configured to use a CSS Grid layout. Some of the items in the CMS are marked as Featured, and we want these to span 2 columns and 1 row.
See the designer view of this page by clicking the button at the top.
Further notes & script examples are in the CodePen.
Also, see a video demonstration of the setup for reference.