You don't say which MICS version you have. But in any case, that is correct you can only put trunk modules on the 1st expansion card.
The reason there is a skip in DN's is because of the B2 DN's on the KSU and existing expansion ports including the ones reserved for the ports being used by the trunk modules.
KSU + 6 port expansion module have reserved 96 B1 DNs 221 -349 (these are the primary DN's that you program and call). In addition to these DN's, 96 B2 DN from 350 - 446 have been mapped.
Therefore the 1st B1 DN available to the new expansion card is 447 (why it comes up as 477 I'm not sure or is this a typo?)
The only way to correct this is to restart the system.
One thing you can try is unplug the lowest trunk module (mod 6), plug in your new station mod and let it come up (should be DN's 301-317, turn on set relocation, unplug the station module and re-plug it into the new expansion module and re-plug your trunk module. My only concern is you may lose the programming on the lines in the trunk module. Not sure if this will work but's worth a shot.
Or you can use Change DN old 477 new 301.
Marv ccna
Sales, Installation & Service for Norstar, BCM & Allworx
Serving Ottawa and Eastern Ontario since 1990