You should definitely be able to do this, but the method varies a little bit depending on the exact set up that you have.
Which smartthings hub do you have? Samsung connect? The original smartthings hub? The new ADT/smartthings panel?
If you have the ADT/smartthings panel, are the motion sensors the dual logo sensors? Or? ( brand and model of the sensors)
And when you say “away” mode, do you mean the “away” mode which is used for lighting rules? Or do you mean the “armed away” value which is used for the security functions?
Also, are you and your wife the only two users on the account? There aren’t also kids or anyone else?
Sorry for all the questions, but as I mentioned, the answers to those questions do affect the method for setting the mode. 