First of all I'd like to thank Doctrine Dark for making and continually updating this mod. He/she has been adding new class compatibility for 4 years now but its not gonna last forever. Here is how you can add new class compatibility by yourself. I'll be using the tempest as the example because I saw comments on this mod and unlock all skill comment page asking for the tempest compatibility.

Step 1: Find the mods folder, usually it is on steam/steamapps/workshop/content/262060
Step 2: Open all skill to all class folder, the default name is 2188081793 iirc
Step 3: Open heroes folder inside 2188081793, inside this directory are the new class folders
Step 4: Copy and paste any folder inside heroes folder then rename the copied folder to the new class you want. I'll rename that copied folder into tempest since that is the class name of The Tempest by Vintage.

To find the class name of the new class you want to add you need to open that new class mod folder and go inside heroes folder. The name of the folder inside heroes folder is the name of the class.

Step 5: Open the folder that you just created and rename, inside the folder there is darkest files.
Step 6: Rename that file into XXXX.override.darkest (XXXX is the name of the new class you want to add, so for The Tempest the file name should be tempest.override.darkest)
Step 7: Go back to the root folder of this mod and open the modfiles using notepad
Step 8: copy and paste this heroes/XXXX/XXXX.override.darkest 103
Step 9: replace XXXX with the name of the new class you want to add.
Step 10: Save the edit

This is how I added new class compatibility for new class such as tempest, Yujin and onikenshi. Be warned tho any update to this mod will overwrite/erase your edits. Aside from not updating until Doctrine Dark officially add compatibility for the class you've added by yourself you can circumvent this problem by editing the title of this mod by opening the project.xml and changing the published field to 0.