There are quite a good number of Civil Servants every year, who transits from other Central Government Grade A jobs to Civil Service. So its quite normal and usual and if you wish to go for the UPSC CSE examination after your UPSC CMS then you can obviously go for it and you can quit your job for civil services.

For more information UPSC CMS consider the link provided below:

https://competition.careers360.com/exams/upsc-cms

For more information regarding UPSC CSE consider the link provided below:

https://competition.careers360.com/exams/upsc-cse

I hope this helps. Wish you a great day!