How to add characters to a game? Text characters... get in, please.

[Tsubaru]

Not sure if I use the right names, but, anyway...

If I want to translate DC games to my language, there is a way that I can add some more characters to it?
Shenmue for example, which has subtitles.

For example, in my language, we have this characters:
ç, ´, `, ~, ^ (to make this [ê, á, é, ú, ã, õ])
And on...

How can I add then to the(don't know how it's the name, "table of characters?"), so the game can recognize this characters and display then.

Is this clear? Well...

Anyone?