Hi. Just to address the pellets or monkey biscuits...some people do use a high quality pellet (ie 'cereal' posted on thepetglider.com) as something to leave in the cage during the day. It should be tossed out every day so that it doesn't spoil. That choice is up to your gliders, do they every get up in the day to snack? Some do, some don't. Mine never do. You also wouldn't want to put it in before they fall asleep or have it in when they wake up, just to avoid them possibly going for that first instead of their staple. The pellets are a LOT of filler.
I use monkey biscuits maybe once or twice a week hidden somewhere in the cage for foraging. Some nights they don't touch it, other nights it's all but gone. Basically I feel that the biscuit gives them something good to chew on for healthy teeth. Again...it doesn't add anything value to the staple diet itself.
Adding a supplement to any of the approved diets is absolutely never recommended. The diets should be made to the exact specifications with no additions or substitutions. I understand your concern that they aren't going for their higher calcium fresh foods, but it would be better to consult the lists of foods with the nutrient break downs to see if you might be able to introduce some other foods that might interest them more.
Many folks using the OHPW will have detailed advice for your concerns. It is my understanding that the mix should have a great deal of the nutritional needs met in it's own form with the fruits and veggies as the 'supplement'.
I'd post the link to the food nutrient breakdown sheet, but I've just realized the time. You may search in the tab above or ask if someone here knows where it is off hand.
~Cheers!