Yes you can, but it depends in what version: if you are using diplomacy 3.3.2 ( for warband 1.134) then you need this , but then you must play it from the start , without old savegames...
This is reason why i begin from the start, and why i created my AAR...
It's your choice, but i recommend to you that you install diplomacy, but without that update, then you should play with your old savegames but without marrying claimants or companions... It adds you too much new features when you are king or vassal