"Portent" itself is already a complete word meaning an omen or sign of something about to happen, often something momentous or ominous.
However, you could use prefixes like:
* Pre- (meaning before): This wouldn't change the meaning of "portent" significantly but could imply a premonition or an early warning.
* Un- (meaning not): This could be used to create "unportentous" meaning not ominous or not having the nature of an omen.
* Mis- (meaning wrongly): This could be used to create "misportent" to indicate a false omen or a misinterpreted sign.
Keep in mind these are not commonly used words, and their meaning would depend on the context.