Here are two lego parts designed for adapting servos an stepper motors to the Lego system.

• Support for servo Futaba S3003
• Support for Step motor 5V 28BYJ-48

These other parts are adapted form existing models :

• Stepper shaft adapter : modified (shortened) from towe original design
• Servo shaft adapter : design from the web (lost reference)

Project on gitlab: https://gitlab.com/hmt63/lego-motor-adapters