You mentioned that the two leads are “normally closed”, but then say that when the motion sensor senses motion it closes the connection…which would be considered normally open. Do you know if this is a dry contact closure?
The EZIO6I requires 3-30 volts to trigger an opto-isolated input to send out an INSTEON command. Being that you want to ensure that you keep the 90 ohm resistance between the two leads so the sensor doesn’t look like it’s tripped when it’s not, how to connect the motion sensor to the EZIO6I and then to the alarm panel could be a challenge.
If you could post a link to the DSC manual and let us know exactly what you’re trying to accomplish, we might be able to help further.