Yes we have that word يكون But since we conjugate the adjective based on the subject and the single and plural, we drop it, it us not necessary.
Sam is generous You can say سام كريم(most used) Or سام يكون كريما(almost not used)
Sam and George are generous سام وجورج كريمان Or سام وجورج يكونان كريمين
Reem us generous (reem is famle) ريم كريمة Or رييم تكون كريمة
In the all above first sentences, you can see the adjective changes corresponding to the subject So it us nit necessary to use "يكون" at all
Actually if you say سام يكون" كريماً," an Arabic speaker would probably understand that "Sam is becoming generous," (which also means that Sam wasn't generous before,) or otherwise the sentence could be taken as a word for word translation of a sentence that has been built in another language rather than Arabic!