You can pin a particular user as a new column if you use Mastodon's Lists feature to create a list with just that user on it, then pin the list as a new column.
More info about using Lists here:
https://fedi.tips/how-to-use-the-lists-feature-on-mastodon/
Once you've got the list set up with just that account on it, open the list and click the top right slider icon, then select "pin".
(To access Lists on the advanced interface, click the ☰ on the left column, then "Lists" on the right column.)