Well, I was only responding to the two-sentence question you asked, so your lawyer knows more about the situation than I do.
In any case, you can arrive at an agreement any way you want. You can use a mediator, lawyers, or your cousin, or none of the above. A separation agreement is just paper with words on it that two people sign. Anyone that can use a pen can write an agreement. The only suggestion I make is to have a lawyer review the agreement before it is signed.
There must be some reason why your lawyer is taking that position. Are you already divorced? Is there a child protection issue?
Also -- I don't know what you mean by "legalized". There's no such term.