Agreed. In Texas you could have a settlement agreement that is not incorporated into the divorce decree but still operates as an enforceable contract between the former spouses. Without incorporation the family court would not have the power to issue a DRO on anything not incorporated so it wouldn't make sense to leave the retirement accounts out of the decree itself or an incorporated settlement agreement. Usually in Texas I expect to see settlement agreements left out when the parties are pro se or the settlement agreement includes property of little value and was informally agreed to by the spouses, such as a verbal agreement to split the personal property in the house. Personally, I prefer to draft all financial assets, real estate, debts and high value personal property even if the settlement agreement covers those assets.
To the OP, if both spouses agree to the amendments then it should not be too difficult to find a local family lawyer to draft the amendments and put it before the appropriate judge.