I got the confirmation from the internal team on this.
Can an existing AVD host machine be enrolled to Intune?
Unfortunately, at this point, we only support enrollment to Intune during deployment for session hosts (i.e. for new hosts).
However, we understand that this feature is important for you. Please post this as a product feedback here. This will allow the community to up-vote and for the product team to include into their plans. We also have the "Give Product Feedback" button at the bottom of AVD documentation to take you directly to the appropriate feedback page.
Hope that helps.
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And, if you have any further query, do let me know in the comments.