I’ve tried adding it in the php but it ends up appearing after the grid page’s layout (at the bottom, side).
That’s probably because you’ll also need to modify the width of the main column to accomodate the extra column. You’ll also want to put the new width within a media query so the theme remains responsive.
You can learn more about using media queries that target certain screen sizes here:
https://en.support.wordpress.com/custom-design/custom-css-media-queries/
https://css-tricks.com/snippets/css/media-queries-for-standard-devices/
https://webdesignerwall.com/tutorials/responsive-design-with-css3-media-queries
You’ll need to do some math to calculate the new main-column width, depending how wide you’ve made the sidebar.
If you need further help, please provide a link to your site so we can have a look directly.