Can we use aluminum channel for the front and back of the robot instead of the KOP aluminum plates, or is this a violation of the pre-designed base rule?
As long as you didn’t design the piece you are using before kickoff then it is fine. Otherwise you are free to modify it as long as you confirm to the other rules, such as total weight, transport size, prohibited materials ect.
What pre-designed base rule?
Did you mean manufacture the piece before kickoff? Teams are allowed to design things in the off-season and then build them during season, see almost every swerve team out there (Except 2481 last year but they were special).