Parsoid has its own extension API - see https://www.mediawiki.org/wiki/Parsoid/Extension_API.
The FeaturedFeeds and GWToolSet extensions need an update to work directly with Parsoid so that we can switch Wikimedia wikis to use Parsoid instead of core parser in late 2021.
The Parsing Team will work with you as required.
Looking at the code, it looks the exposure is fairly minimal (in FeaturedFeedChannel.php for FeaturedFeeds extension and Forms/PreviewForm.php for GWToolSet) and once ParsoidExtensionAPI adds support for transformMsg, this work should be relatively quick and fairly straightforward. Since this is an extension that doesn't register any parser hooks and simply requests a parser object and parses some wikitext, it is possible that the abstracted parser base class might satisfy the contract and this work might be a NOP. But, for now, I'm leaving this task in place just in case there is work to be done here.