I'm not exactly sure you can modify the collar without resorting to major, and I mean MAJOR surgery. You could, however, place small velcro patches on both the inside and outside points of the collar and then modify the jabot by removing the band and sewing a tab to the centre of the jabot and a placket down 2/3s of its length. Sew two button holes in the placket that will line up with the top two buttons on the uniform jacket.

Before the collar is hooked up, attach the tab to the inside of the collar. Hook up the collar and fold the jabot over, pressing it against the velcro on the outside of the collar. Now button the placket to the top two buttons of the jacket, and everything should be okay. (The button placket is necessary to prevent the jabot from blowing around and looking like my Aunt Minnie waiving bye-bye to a train with her hankie.)

For civilian wear you should also attach a stand up collar to the inside of the jacket collar. You can obtain these from any clerical supply house for a few quid each. They will prevent the collar of the jacket from chafing your neck, and will allow you to fit miniature clan badge "collar dogs" to the collar as well.