You can try by putting chopped up pieces in with their current main diet. Hanging leafy greens in their cage or clipping them to the side is a good way to offer them. Do your bird's eat mashed up hard boiled eggs or bread ? You can add veggies to either of those. Lot's of good hint's and advice in the diet and nutrition sticky's, most of all patience is required, we have folk's here all the time that hang tough and get the breakthrough eventually...
He came down from Heaven unto this earth below
He came down from glory and praises untold
He came down to man fashioned in their way
He came down to rescue, He came down to save
I find that my budgie is really motivated to try new foods when he sees me eating them in front of him I'm not sure how strong of a bond you have with your bird, but if he/she is fairly close with you then that's what I'd recommend. My birdie Prince is always looking to share when food is being eaten! As suggested above, leafy greens seem to be extra appealing to birds, especially when placed in strategic locations (near favorite perches, by main food, toys, etc).