The answer is yes!, Dominus Class Knights can be used in the Horus Heresy. We don't have rules for them right now, but with Forgeworld Open Day coming in a month we are certain to have them then. Plus a Sneak Preview.... above. This is what was said this morning from Forgeworld over on the Community site.
via the Warhammer Community
https://www.warhammer-community.com/2018/06/25/25th-june-new-forge-world-knight-rules-and-patternsgw-homepage-post-2fw-homepage-post-1/
"With both Dominus-class Knights now on shelves, loads of you have been asking if you can use them in your Horus Heresy games.
Now, we have an answer – yes!"
"In fact, the team got a bit carried away – if the guys at Forge World love one thing, it’s massive war machines. You won’t just be able to use the Knight Valiant, Knight Castellan and Armigers in the Horus Heresy, but a whole range of new Knights armed with all manner of esoteric weapons from the Age of Darkness and based on these new designs."