Most Technical and Vocational Colleges in Kenya require that you score atleast a D- (minus) and above in KCSE to do an Artisan course and D (plain) and above to do a Craft course. However, there are some Artisan courses on KUCCPS portal that allow E students to apply. You can visit https://students.kuccps.net
and apply for Artisan courses when applications are open.
Unfortunately, you cannot do an Artisan course then proceed to doing a diploma course.