It is possible but carbon costs good money then you have to pack it to a certain grade and density on a vibrating table plus they aint easy to get into and after all that there is no guarantee you have a perfect product that will match your fan cfm or keep smells at bay.
Mainly buy a new one but i guess some might have achieved a refil. Use to be companies would offer refils but i aint seen that for a long time.
I keep repeating this info and no one says much different but still worth a google, maybe rhino or some of the more exspensive filter companies can help or guide you through the process. Think its 3-4mm grade activated carbon you need.....
Ah fuck it by this stage most have paid for a new one. Certainly worth downsizing if possible to save a few bucks come filter replacement but ive never quibbled as they last for a few grows.
Never needed them on smaller grows as smoking weed normally masks the smell but as a grower there is certainly a distinct difference between the smell of smoked weed and fresh plant bud.