I recently stumbled across this excellent looking embroidery machine build on thingiverse. It would not be hard to modify the multibot to drive an embroidery addon.  You would either need to get rid of the auto squaring and just clone the Y axis steppers to free up the A axis, or you would need to unplug the Z axis stepper and use it to drive the sewing machine needle.

It is also possible that you could modify the sewing machine so it cycles once every time you send it a pulse, that way you could skip adding a stepper motor to it all together.  It is possible that more modern sewing machines can operate in this mode where a foot switch cycles the needle down and up once.