I've dropped in media bags outside of the micron filter, and I've filled them w/ crushed shells (to raise kH), carbon, etc.
Whatever you need and can fit will work. BTW, the HOT Magnum has been an awesome alternative to canisters with a hose. You need less room than the large HOB filter it replaces, but it sure is quiet and moves a lot of water.
v/r, N-A