I added anything in ziatype. In fact my favortie additive is platinum (chloroplatinite salt) as it reduces the Drange of ziatype adn treuly improve the separation in the low light. should mention that in ziatype the addition of platinum give warmer tone . Another good one is gold in low quantity it as the same effect as platinum but give colder tone .
I should also add that ziatype may be control by many factor : humidity , chemistry , and even developer. Sullivan and Weesse demonstrate the use of a method where the humidity is control ( 50-65% Rh), in this case you control the color by the chemistry ( gold , ferric salt, platinum, tungstate, etc) . hope this help