Create your Key_All name with this formula:
=INDEX(KeyA,INT((ROW(OFFSET($A$1,0,0,COUNTA(KeyA)*COUNTA(KeyB)))-1)/COUNTA(KeyB))+1)&INDEX(KeyB,MOD(ROW(OFFSET($A$1,0,0,COUNTA(KeyA)*COUNTA(KeyB)))-1,COUNTA(KeyB))+1)