Hello,
You have two option:
- Use the "Swap Q/A" to use the deck "backward."*
- Create a new card model for the reverse question on Anki on the desktop. As far as I know you cannot create card models on AnkiDroid**.
Mostly (my impression is that) AnkiDroid is meant as a companion app for Anki on the desktop so some of the functionality is available only on the desktop, like editing the card models.
If you just want to do this on your existing decks, you can also email me privately your decks and I am happy to do that for you.
-Flavio
* This is available in the 1.1.3 official release. I am not sure if it is there in the 2.0 betas (I have not found it after a brief inspection).
** This might have changed with 2.0 betas, again I could not find the option.