Seems easy enough with Discourse, even on mobile.
What do you mean by manual?
The way I did it for this post is just select the text and hit Quote. Then added my reply. Then selected new text and hit Quote again. You can even quote multiple people or even from other posts.
Check this:
Cool, right? I just minimized the reply box (on mobile) then went to another post and did another Quote and it dropped into my ongoing reply.