My question was closed as "non-constructive", so I asked it in other place (Freenode) where I got the answer which I want to add to superuser.com.

What should I do?

1. Edit the question, including the answer as section of the question itself;

 1b. "Re-brand" the question to suit the answer more + include the answer into it;

2. Vote to reopen, then add the answer normally;

 2b. "Re-brand" the question and vote to reopen

3. Add the answer as comment to the question;