If so, how? I noticed there is no "check mark" next to the answer that I found usefull (more so than the similar question where I was told I should find my answer)
Yes, you can. There's nothing to stop the asker to accept an existing answer on their own questions regardless if they're closed or not.
However, you can't accept any answer on questions not owned/asked by you.
On the other hand, feel free to upvote any answers that are helpful to you.