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.
A "sus2" chord does not have a third. It is spelled 1, 2, 5. (In the key of A this is A, B, E)
An "add9" chord has a third. It is spelled 1, 3, 5, 9. (In the key of A this is A, C#, E, B)
"2" and "9" are the same note. In the key of A, this note is B. Both an 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
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.