Adding a char requires being able to mod the GS and libs, and match their exact sizing up and some other parameters.
Nobody has pulled this off yet afaik. The only way I think you would be able to do this is with the source floating around, you could potentially learn how to add classes that way. That source is 15 class iirc.
Even modding skills is very limited. I'm not convinced you could add skills to 1345 without the actual source. To work with the libs you use ide and unpack and edit. It's decently easy to make minor changes to skills, but one of the restrictions with editing is we can't change the size. Adding a new skill would change the size. You'd have to figure out how to tell the GS to load that lib after it has changed. You are working with compiled files, it's not the same as source and you will be restricted. Even more so depending on which tools you use.