Can more material be added after Medpor implant?

The perception that Medpor facial implants are difficult to remove is overstated. I would not stack other implants on top of the ones you have because of 'removal difficulty' concerns. Get the old implants out and replace with a whole new implants that better suit your aesthetic facial desires. If the maxilla is flat that can augmented with implants at the same time.

Thanks for your question. 
There are a few options to address volume loss in the midface.  In the right patient, medpor implants can be great. In your situation, these medpor implants could be exchanged, however, it might make more sense to use other means of volume augmentation. It is possible fat would be all you need. Other options would be calcium hydroxyapatite granules or sculptra. Without a full examination, it is hard to make a recommendation, but I suspect fat may be the most ideal given that it is less invasive. Best of luck!