What I want to do is to change letter spacing of a readymade font and save it as a new ttf file.
Lets say I want to use Myriad Bold with 0.05em letter spacing. So I want to edit (add) 0.05em spacing on readymade spacing.
Is that possible, and if it is, how can I do that easily?
Simply select the text in whichever layout app you are using (InDesign, XPress, etc.) and increase its spacing. If you do not know how to do that, please refer to the documentation of the app you want to use your font in.