Sure, via events. Add the following in the EVENTS.TXT:
@IF
NEGOTIATION
talker=ANYBODY
talkertype=HumanOrComputer
listener=ANYBODY
listenertype=HumanOrComputer
@THEN
@ENDIF
Please note that some scenario creators have said that this doesn´t work with all scenarios. In some cases it´s neccesary to replace talker and listener with the civnames. That means that you have to create sepparate events like the one above for EVERY PAIR of civs you have in the game, like this:
@IF
NEGOTIATION
talker=Romans

talkertype=HumanOrComputer
listener=Greeks

listenertype=HumanOrComputer
@THEN
@ENDIF
AND
@IF
NEGOTIATION
talker=Greeks

talkertype=HumanOrComputer
listener=Romans

listenertype=HumanOrComputer
@THEN
@ENDIF
However, the first event works with my scenarios...
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Civ2000