I got it now, yet, it is a 3 a steps solution and and the casual user of other forms will find it annoying...
I think that the correct way is to select part of the source and quote and no to copy, paste & quote.
Don't you agree with me @robert?
The feature you're talking about is also known as Multi-quotation. And yes, this is already in our to-do. But that's a hard JavaScript solution which requires a lot of time. WordPress and wpForo use TinyMCE rich editor, that's not easy to integrate such JS function in this editor. This kind of functions are being added in major or in middle versions. We can't do a simple change and add it.
@robert, I understand. I'm not criticizing, I just want to help making wpForo the best forum platform and injecting my comments on that.
[Off topic - and as you noticed, I've included the '@' in-front of your name as a reminder that tagging is also important...]
Have a great new year!