can I attach large filters to my RO/DI unit?

[glarior]

I have a buckeye field supply 100gpd RO/DI unit with extra carbon filter. I make ~5 gallons of water a day and more when I need it. My media burns up really quick since my water is horrible and contains 450+ TDS and lots of chlorine. I have three 4.5"x20" filters with removable cartridges. Is there a way to use those or incorporate the big filters with the RO/DI unit? I want to buy the media in bulk (anyone know of a good place?) and have it last longer. Seems like I change those small filters every 1-2 months. For the price of one filter change on my small unit I can buy the carbon filter, 5lbs DI resin and sediment filter for the 4.5"x20" filters