In a comment on Laurence Payne's answer, you wrote that you want to know if you can consider Emadd♭9 "a key that has some chords in it." The short answer is no, because Emadd♭9 is a chord, and chords are not keys; chords can belong to keys or have a function within a key, but there are many possible keys that can enclose a given chord. So while C major is the name of both a chord and a key, there are actually several keys in which we can expect to find C major triads: G major, B♭ major, and F minor all come to mind. C major (the chord) has different functions in all of these keys. Likewise, Emadd♭9 can function in several different keys.
What I think you are asking, then, is which keys might contain the chord Emadd♭9. If we prohibit accidentals, there is only one major key that contains Emadd♭9: The key of C major, in which Emadd♭9 is a iii chord. In practice, we usually play simply Em or Em7 as the iii chord in C major, however. If I wanted to play something more colorful than that as an accompanist, I would probably use plain Em9 instead of ♭9 since it's less jarring.
The modes of C major are also valid, of course, so if you consider D dorian, E phrygian, F lydian, and so on to be "keys"—a more accurate word would be tonalities—you get six more scales that contain Emadd♭9.
You might also see this chord in an E minor context. For example, if you wanted to resolve a B7 in a suspenseful way, you could move from there to this Emadd♭9 at the end of a phrase, then resolve to the normal E minor.
Broadly speaking, the minor add♭9 chord is very rare because it is dissonant and difficult to contextualize. In fact, in my theory classes I was taught to avoid using the ♭9 over a minor chord even as a melodic passing tone! (There are many exceptions to this, e.g. Jobim's "Corcovado.") That's not to say that you shouldn't use this chord, but rather that its applications are pretty narrow, and you'll benefit greatly from simply studying how other composers use minor ♭9 chords and playing around by pairing Emadd♭9 with other chords that you know.