Ah yes, that's not possible as far as I know. But it's still easy enough to quote the paragraph and delete what you don't need, leaving only the sentences that you want to quote. Although I find this need of yours a bit unconventional, since when I quote multiple people or only one who also quoted others in his message, I prefer my reply showing all the nested quotes, so who's reading or is interested in the discussion doesn't need to scroll up and look for references, because they are already there. In sum, you like more xenforo's ways, while I don't, and I prefer WBB approach. My that's just my 0.2 cents, and I understand your needs here.
Anyways, maybe there is a way to select and quote single sentences you'd need to include in your post, but I haven't found it yet.