For now there are separate io objects in Breakout for DC motors and Stepper motors. However because the Adafruit motor shield used a shift register in driving the motors, direct control of the motors on that board is not currently available.

However, control of the AF Stepper Shield will be possible using the new ConfigurableFirmata: https://github.com/firmata/arduino/tree/configurable/examples/ConfigurableFirmata.
ConfigurableFirmata is still a work in progress so it is not well documented at the moment.

There will be a couple of different ways to approach adding support for the adafruit motor shield. One is adding a new feature class to the utility folder (in the configurable branch) that wraps the af-stepper-class. The other is using the Shift feature class (in progress but should be added soon) to implement a custom feature class that has a similar API to the AF stepper class. In either case a new IO object in Breakout could be used to control the stepper shield.

I may have time to add an example for driving the stepper shield to the configurable branch in Firmata and to a new branch in the Breakout repo on github in a couple of weeks.