garage band for ipad change standard chords in smart instruments

I would like to change the 8 default chords in the smart instruments (there is this possibility, for example, in the "classical guitar" free app).
For example I may have the need, while playing a song in the key of C, to replace the Bb (available) with an E. It seems that is not possible... also it would be good to have the possibility to introduce altered chords like an E7 or a D7 while playing in C...
has anybody had the same problem?