2 and add9 chords
The difference between these two chords is not what you might think. It is the third scale tone that makes the difference.
"2" and "9" are the same note. In the key of A, this note is B. Both Asus2 and Aadd9 have a B note, but only the Aadd9 chord has a C# note (which is the third). So, it really is the third note that makes the difference.
A "sus2" is sometimes called just "2" - for example, "Asus2" and "A2" are the same chord
2 chords are constructed with the 1st, 2nd, and 5th notes of the major scale
A sus2 chord could always be substituted for an add9 chord.
An add9 could be sometimes but not necessarily always be substituted for a sus2 chord, because it's possible the sus2 chord could be played in a situation in which the major/minor is ambiguous and we would not want to add a Major third in some cases.