Watch the Drop 3 Voicings: online guitar lesson by Brad Carlton from Guitar Lab: The Golden Arp
The drop 3 voicing of the Add 9 chord in 1st inversion is used less frequently because the 9 (2) is the lowest voice. This chord can be thought of as a root position major triad with the 9 (2) in the bass. Another common name for this chord if labeled as the 4th voice being the root is: 9sus4 (or sometimes 11).
It is derived from the root position close voiced Add 9 form. Use caution when playing this voicing in the lower registers and remember to hear it in reference to a root in the bass (played by the bass player, keyboard, etc.) in order to perceive the harmony as an add 9 chord.