Could someone show me how to mod the game so every player in every political state (war, cold war, etc...) could trade systems with each other?

Hey mattking!

The star system exchange diplomatic term is defined in [ES2 folder]/Public/Diplomacy/ForeignAffairsConstructibles[Exchanges].xml, and is declared in 3 variations:

ColonizedStarSystemDealForAlliance, ColonizedStarSystemDealForTruce, and ColonizedStarSystemDeal.

If you override the last one (see the modding guide for more info) and remove the following part, the action will be available for everyone all the time (except Vodyanis because they have Arks):

<DiplomaticMetaPrerequisite Flags="Prerequisite,Discard" Operator="OR">
    <DiplomaticRelationStatePrerequisite>DiplomaticRelationStateColdWar</DiplomaticRelationStatePrerequisite>
    <DiplomaticRelationStatePrerequisite>DiplomaticRelationStatePeace</DiplomaticRelationStatePrerequisite>
    <DiplomaticRelationStatePrerequisite>DiplomaticRelationStateAlliance</DiplomaticRelationStatePrerequisite>
    <DiplomaticContractContainsTermPrerequisite>ColdWarRelation</DiplomaticContractContainsTermPrerequisite>
    <DiplomaticContractContainsTermPrerequisite>PeaceRelation</DiplomaticContractContainsTermPrerequisite>
    <DiplomaticContractContainsTermPrerequisite>AskAlliance</DiplomaticContractContainsTermPrerequisite>
</DiplomaticMetaPrerequisite>
<DiplomaticRelationStateEmpirePrerequisite Flags="Prerequisite,Discard" Responsible="EmpireWhichProvides">
    <PathPrerequisite Inverted="false">EmpireTypeMajor,AffinityGameplayVenetians</PathPrerequisite>
</DiplomaticRelationStateEmpirePrerequisite>

I'm very grateful for you quick and precise answer. With this I was able to do it. Good to see that some games has such a great community. Again, thank you friend.