Firstly, I hope to enter the undergraduate school to increase my last GPA (though I have a BSN), so do I go into the school by transfer? What grade can I get in? I know the school set the minimum GPA for transfer students, do they consider the student's job experiences?
I don't know how applicable my advice is to Korean educational systems, but......
I went back as a non-degree student. As a non-degree student, I was able to take any class I wanted and not worry about pesky pre-reqs. They did want my transcripts, but they were not weighed in their admission decision, since I like you have a BSN. Also, as a non-degree student, I had no class ranking, which facilitated taking any class I wanted as stated earlier. Over here, Most schools are just glad to get your money, and will let you in with any GPA, if you already have a degree.
secondly, when CRNAs usually retire? I wonder how long I can work as a CRNA after graduation from CRNA school.
I can't tell you what average retirement age is, but I can tell you that at my hospital we just had a woman who was 68 retire form a long career as a CRNA. I have heard that there is going to be a shortage of CRNA's in the US in the near future, do to retirement of the current CRNA's.
Good luck in your quest to become a CRNA.
I assume by your questions, that Korea uses CRNA's then?