If both of you agree, then it can be signed by the judge as an order. But you should include a paragraph or statement in it that states something like: 'If both parties agree in writing to anything beyond the scope of this agreement, it should be considered equal in scope with this agreement and equally enforceable.' Or something comparable in legalese.
Talk to the mediator who's helping you with the
parenting plan...it's very possible that they may recommend having the plan filed with the court, too. Really, the whole thing is absolutely worthless unless it's made into an order. Let me guess....she's agreeing to just about everything right now, right? Because she has NO intention of following it. That's what you need the mediator for....tell them that you will NOT sign the agreement unless it is filed with the court.
But be prepared...mediation may come to a complete standstill at that point, especially if she refuses to agree to it and having it filed with the court. At that point, you will have to go back to court to get the judge to work it out between you and the ex. THAT is probably the hassle she wants to avoid...which she can if she agrees to the plan and having it filed.