After many hours of looking into how to get addon hats working with hair, I have figured it out! Edit mp_creaturemetadata.ymt (or whichever DLC you use (Ex: mp_creaturemetadata_heist.ymt)) Add more pedPropExpressions items and copy-paste the 19th one (in the default one, if DLC then the last one and then follow the rest) and change the number from <pedPropVarIndex value="19" /> to 20 and etc! So you won't have addon hats have hair clipping through your hats. (What each item looks like in <pedPropExpressions itemType="CPedPropExpressionData"> ) What tracks and ids are, is still unknown.
<pedPropID value="0" />
<pedPropVarIndex value="19" />
<pedPropExpressionIndex value="0" />
Make sure to go into mp_m_freemode_01.ymt(or mp_f_freemode_01.ymt) (or if it's a custom DLC one) and change the following
<expressionMods>-0.5 0 0 0 0</expressionMods> (which Credit to grzybeek#9100 for finding this out and for the image) is used to tell the game to which hair it should be to hat under the hat !