Close to one of the other answers, it is quite common to use
- EN: meaning Editor's Note
- AN: Author's Note
- TN: Translator's Note
- PN: Publisher's Note
but make sure to write them in full the first time they are encountered in your book
Another solution is to use a different kind of symbol (for instance numerical for one kind and alphabetical for the other) and different typography (not the same font, block of notes separated by a short rule) for original notes and for added notes. This is sometimes used in critical editions of ancient texts: one separates notes about the "edition" : choice of words and readings, interpolation of missing words, notes about the "translation", "meaning", "context".
This can soon become really complex or distracting, eating up a lot of space.
If you want to do a good job of it I would advise you to choose carefully your word processor program. For this kind of job I use TeX or XML based systems.
If you are both the original author and the translator, why don't you decide to adapt your own text as a new edition incorporating as much as possible in the flow of the sentences ? It will be easier on the readers. Just keep invisible (non printable) notes for yourself to keep track of your changes and your evolution.
As many people, I feel that in general books, except for references and unescapable precisions needed for the intelligence of the text by someone who is not the original author, footnotes are best avoided.
Footnotes were particularly useful when texts were typeset using lead characters and a press: it allowed to change and extend text without reworking the copy too much, it was the equivalent of an afterthought of the author.