Wallets – Which Wallets Allow Generating Multiple Ethereum Addresses?

You're going about this in the wrong direction.

You cannot construct arbitrary seeds from existing keys, nor can you add existing keys to an existing seed.

A seed->address key process is one way, and you must start with a seed. Any existing keys will have to be handled separately, and will not be related to the seed in any way.

There is a standardized method for generating multiple coin addresses from a single seed, which you can find under BIP-44. The list of registered coins can be found at SLIP-44.

There are a number of libraries to work with these specifications, for various coins. As long as you provide them with the same seed, all of your addresses will originate from that seed.