I've got a font called Lato, and I need to add the characters İ, Ğ, ğ, Ş and ş. How can I do this?
1st photo is the original. 2nd photo is the same font that I upgraded:
It is possible to add extra characters to fonts, but it isn't something for the faint-hearted.
The free and open source way to do it would be with Fontforge. However Fontforge, like other font editors, is complex enough that simply exporting your modified font to .ttf or similar is in itself a complex undertaking. It is a very steep learning curve. What's more, making a font that's compatible with Mac and Windows and hinting it properly is a fine art.
Other considerations include:
Note: if you do create fonts, test them on Chrome for Windows! Chrome on Windows uses Windows GDI ClearType which will render a poorly-hinted font badly and is a good litmus test for your font.