Hi all,
You can make Akadama (or any other substrate with cation exchange capacity (CEC)) add dGH, you just soak it in a solution with a lot of Ca++ or Mg++ ions and the H+ ions it has in its exchange sites naturally will be exchanged for the divalent cations. You could use CaCl and/or MgSO4.7H2O as your ion sources.
As well as the CEC, clay minerals also have anion exchange capacity (AEC), so if you add CO3- ions (from KHCO3) you will raise dKH as well. As a general rule it is easy to add salts to water, and much more difficult to remove them.
As "pepedopulus" suggests coral sand (a biogenic aragonite form of CaCO3) will add both dGH and dKH, but another cheaper option is "Oyster shell chick grit".
cheers Darrel