According to https://fishshell.com/docs/current/cmds/set.html?highlight=environment
You'll need to use the set command to set the variables and explicitly export them.
According to https://fishshell.com/docs/current/tutorial.html?highlight=configuration (search deep in the document for "Startup (where's .bashrc)" the file you need to edit is ~/.config/fish/config.fish)
So putting those two together, add the following to the end of your existing ~/.config/fish/config.fish (or just create a file containing only the following if that doesn't already exist):
set -gx KPAX_ROOT <your path to KPAX installation>
set -gx PATH $PATH "$KPAX_ROOT"/bin
Alternatively, it might be even easier to just create the needed variables as "Universal":
set -U KPAX_ROOT <path to KPAX installation>
set -U PATH $PATH "$KPAX_ROOT"/bin
(You should only have to execute this once and it should update fish for eternity on your behalf according to the documentation).
I don't have and haven't used fish, but hopefully these references to the fish manual and the suggestions I provide will get you where you need.