This is a regular verb. It follows the conjugation pattern of the verb falar.
Present:
A Ana sussurra a resposta.
Ana whispers the answer.
Preterite:
O Miguel sussurrou algo no ouvido dela.
O Miguel sussurrou algo ao ouvido dela.
Miguel whispered something in her ear.
Conditional:
A Clara sussurraria o segredo.
Clara would whisper the secret.