When you create a Proton Pass alias you can manage it using SimpleLogin too, but you can’t see or manage SimpleLogin alias using Proton Pass apps/extensions.
Update: There is a custom DNS option in ProtonVPN, but it supports only IPV4 and it's only in the Windows client.
If only Proton supported this function natively. Sadly ProtonVPN gets little love from Proton, by far its most underdeveloped service.