This has just happened to one of my answers, so I thought I'd ask here whether people think it's OK.
I posted an answer to a question about physical units appearing in exponents and the like. Shortly afterwards, a whole paragraph of additional explanation appeared at the end of my answer. The editor had around 4500 reputation points.
I don't disagree at all with what the additional material says, but it's certainly new material. Good material, but an additional point beyond what I'd said.
I dealt with it by putting it in square brackets with —added by . . . to credit the source. But is inserting substantive new material into an answer this way considered appropriate? I thought we were supposed to avoid it. (In fact I nearly had an edit rejected which consisted of adding something the questioner had asked me to—he'd said it in a comment but didn't feel confident in adding it himself.)
About a third of the answer now consists of the added material.
Edit: I've now replaced the inserted material with an explanation based on it.