Yes, you are allowed to use a LEGO model just to define the start position of the robot.

The LEGO model is seen as a part of the robot.
The model must be disassembled when the building period starts, just like the rest of the robot.
The robot must remain on the field during the whole run, the team cannot touch it after the start.

Teams need to follow rule 6.10 (no information is provided to the robot about the position of elements on the field using this LEGO model).
The LEGO model and the robot together must be inside the 25cm x 25cm x 25cm when measuring.