Yes, I follow what you are asking.
Yes, CRNA's are also considered to be ARNP's. In fact, in Kansas I have to have both a CRNA license and a license as an ARNP to practice. But the core knowledge required to be a CRNA is different than that required to be a FNP, so the course work is completely different. I know at least one FNP here that would like to be a CRNA, and is planning to go through the full CRNA program. There are no "certificate programs" or diploma schools in the US that I am aware of, but then I have not checked.
The only exception I can think of might be at a university that offered both CRNA and FNP programs, with some core courses that were common (i.e. pharmacology). If you were an FNP with a degree from that university, then you probably would not have to retake those classes. Additionally, if you had already taken, for example, nursing research or nursing theory elsewhere, you probably would not be required to take it during the CRNA program (check with school directors to be sure).
Something I just thought of: Check with the anesthesia schools where you intend to apply. Some schools will allow you to take some of the core courses without actually being enrolled in the program at the time. This can reduce your workload during a very intensive course of study.
I know Newman University allows this. There are several classes, including pharmacology, physiology, pathophysiology, anatomy, chemistry, and physics that you can take there without actually having been accept to the program. One guy I know took everything he was allowed to take before entering the program. Basically, he has to concentrate ONLY on the program content specific courses (Principles of anesthesia, etc) now that he is in the program.
The down side to doing this is what happens if you don't get accepted into the school where you took the courses. Most programs don't allow the courses taught at other programs to count for their program, so some material ends up having to be repeated.