Technically yes, but you will find that it is in no way a CR 18 challenge.
My feeling is that for adding spell using classes to a creature it is more effective to treat it as if there were two separate beings and calculate the EL - so for a CR10 psion and a CR8 mind flayer, the final EL would be, what, 11? That might seem too low, but considering that a CR18 wizard or psion would have 9th level powers to throw around, while this psion 10/illithid only has 5th level powers... I think it is much closer to reality than just adding CR to class level.
After all, a single creature is normally doing one thing each round, while two creatures can help one another, provide covering fire, provide more targets etc. etc.
Cheers