Can you use filler to enhance abdominal muscle? I've a lot of bone around my sternum and would ideally like to fill in some

It's difficult to give a recommendation without more information or photos. Typically HA fillers would not be used to accomplish something like this. You may be a better candidate for fat transfer, which would also give you a more permanent result. See a board certified plastic surgeon near you for an evaluation.

Hi aceform,

Its hard to make a recommendation online without physical assessment.
Fat transfer or sculptra may be an option. It would be best to book in a consultation.

Thanks!
ACM.

Firstly, please note that the sternum is the breast (or chest) bone, which is different from the abdominal area.
It is, indeed, possible to use fillers (whether man-made or your own fat) over the central chest area to minimize a 'skeletal look'.
In terms of enhancing abdominal muscles, I do not feel you should be considering fillers. Even just from a financial perspective, such would not make sense. Better options would revolve around high-definition liposculpture, but it's hard to guide without seeing any of your images.