Using both a stepper motor and a DC motor with Arduino motor shield R3
You are right: Motor Shield R3 can drive two DC motors or one step motor. So, there are 2 (maybe  3) solutions:
1. Have another shield - now you can drive 2 step motors, or one DC motor(using 'half' of shield, and one step motor using another whole shield.
2. Replace your step motors with DC motor. This option can be more complicated, as no precise position control will be possible when you try to use DC motor for steering. (DC motor are hard to select and hold position, when stepper motors are quite easy to 'go to certain position' AND 'hold'). There is also good mechanical skill needed to remove/place it precisely.

optional option 3. Instead of Motor Shield R3 you can try to use cheap Easy Driver to drive your step motor.