Yeah, but this command would just be a restriction, the channel manager has already access to enough restrictions to set on his channel, he (the channel manager) ought to manager his access list properly and add "ranks" to the levels, not adding people with a level higher than 400+ (if not trusted).

The whole point: trust.

Such a command would be pointless since by adding people with 399 level issue is dealt with and new users can't be added by those lower than 400.

Level 400 is define as channel administrator (not to be confused with CService Administrator), effectively if a channel manager does not want someone to have the ability to add users, then simply put, they should not give that user level 400+ access.

Have a look at the full command list at http://cservice.undernet.org/docs/ and you will see the commands are grouped but type of access.