Not possible yet. The devs said it may be coming in the far future. kbin supports this, but it's a bit buggy still and is not yet compatible with Lemmy. I'm considering switching once they turn on federation (so that you can join Lemmy communities from there and vice versa).
This is all still a bit early still, unfortunately. Though I'm sure now that so many people are here development will accelerate significantly.
Edit: People on Mastodon can reply to Lemmy conversations, though. And we can see their comments here.