As long as the speed and duplex are the same, I believe the port(s) should join the LAG. The only hardware limitation I am aware of with LAGs is if you are operating the existing LAG on a 10Gx24-DM module.
If using the 10Gx24 module, port group members can only exclusively consist of 10Gx24 ports.
(This is for the MLX/Netiron platform)
Mike Morey
Michael Morey
Principal Technical Support Engineer
Extreme Networks