HIVE

You can add functions by adding them to a user include file (must be either a .txt or a .j file). Put that file in the tesh/includes subfolder. In the file, just add your full function definition, for example:

function MyFunction takes integer i returns integer
return 2*i
endfunction

The function will then be added to the autocomplete list, the syntax highlighter, it will have calltips and a function list entry (in the category "user includes").

Also note that you don't have to restart the WE if you added new custom functions: You can reload user includes through the TESH menu by "TESH->Reload user includes".

Can I also add keywords with this?

No, only functions are supported for custom user includes (like in the very old first TESH by SFilip).

You can in principle also modify the tesh_keywords.db database where all native keywords are defined, but you will need SQLite to do this.

Since I'm using the old version, I don't seem to have that database. The one I do have is for functions and constants, but doesn't contain keywords.
Were the keywords hardcoded in that version?

What you desribe sounds like a very old version.

Which version are you using exactly? You should see that if you click on the "Help" button. Is there a particular reason that you don't use the latest version?

The one that came with jngp 5d or 5e. I ended up merging those editors at some point, so could've been either one.
It says version 0.7 and looking at the changelog files, the last update was to Horus.
I've been using this one simply because that's the one I've had for a long time and haven't had issues with it.