You can’t add new or custom networks to Phantom manually. Phantom only supports the networks that we've integrated directly into the app, which are Solana, Ethereum, Polygon, Bitcoin, Base, Sui, Monad, and HyperEVM.
If you need to use a network other those supported in Phantom, you'll need to use a wallet that offers this capability.
If you’d like to request support for a new network, submit a feature request at feedback.phantom.com. Our team reviews suggestions there to help guide future updates.