There's nothing really for a moderator to do in these circumstances that you could not do yourself*.
So, it's hard to see what a new specific flag type would be useful for. The other flag types should suffice in all cases I can immediately think of.
The main reason I have a separate account for comments converted to answers is that I don't want these answers associated with my main account. Max Vernon has one of these as well, off the top of my head. There may be others.
The separate account is also useful to show people that regularly leave perfectly good answers in comments how much rep (and badges etc.) they're missing out on!
People that know me will already know my (strong) opinions about answers posted as comments. Honestly, aside from making work for others, I find it a little arrogant to post an 'answer' that will always be at the very top of the page, and cannot be downvoted 😉
All that said, I do agree with you to some extent. It would be nice to convert comments to answers directly sometimes. The reason I don't advocate for it is that I don't like the idea of posting an answer with someone else's name on it. They chose not to answer, so I shouldn't force them to. I don't altogether love that reasoning in all its aspects, but it is a tricky issue.
I am, however, more than happy to delete comments that shouldn't have been comments, so long as no truly useful content is lost, one way or another.
On the whole, the issue isn't out of control on dba.se like it is on some other sites, so I am grateful for everyone's help on that. Particularly those of you that regularly edit useful comment content into posts and flag the original comments as obsolete.
* For people that really don't want to take action themselves, I personally am willing to handle the odd custom flag explaining that they'd like me to convert the comment(s) to an answer, but I can't speak for the other mods. Also, I'd really like not to spend my own time on too much more of this - it is limited like everyone's.