BM Silvia has summarized well what I would like: I would like to be able to connect with a sub-account Protonmail that would be only dedicated to the emails of my Gmail box. Currently, creating a Protonmail sub-account is useless since by connecting with the sub-account, we view the emails of the default parent account.

Example : "arthur.martin@proton.me" is my default Protonmail account.
I would like to create a new sub-account "arthur.martin.GMAIL@proton.me" to see only my Gmail import emails (and NOT "arthur.martin@proton.me" emails).