I think that's a good idea.
You click a little reply button at the bottom of a post (just left of the Like, Dislike buttons) and it copies the user's name formatted in bold with a colon, and the user's post formatted as a blockquote, to the "Have your say" box at the bottom and positions the cursor on a new line below so you can write your own reply comments. If you select a small portion of the user's post before clicking reply, then only that small portion is quoted, maybe with "..." before and after to indicate it's just a snippet.
I wish to also say that I think each post should also be bookmarked with a unique html anchor tag (you can use the user's name combined with the timeserial corresponding to the time (down to the second) of the post. That way you can also include a link in the reply post that takes you back to the source location of the quoted post that you're replying to.