It's certainly possible. 90210 years are different from what we know or understand. Brenda could have became preggers on the season 4 finale; or, when Dylan went to London to visit. Brenda could have put Ade up for adoption, as a baby. Maybe, Brenda doesn't even know that Ade is her child. So, it is definitely possible.
But, you know, it's all speculation. Cool idea though.