Don't see why not. P.61 of the CSM Codex says that one of the options for a summoned daemon to enter play is via a *Champion*, and that the owning player chooses "any suitable model" in his army. All Chaos Terminators have the option of being upgraded to champions, so I can't see a problem with it.
Page 61 of Codex: Chaos Space Marines says that a Greater Daemon possesses "the body of either a Champion, Aspiring Sorcerer, Sorcerer of Chaos, or Chaos Lord". But since there are no mere "Champions" in Codex: Chaos Space Marines, only "Aspiring Champions", "Skull Champions", "Plague Champions", "Noise Champions", "Possessed Champions", and "Terminator Champions". Thus, all of these are equally viable for the summoning a Greater Daemon, including the latter.
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