You would need to own a domain and set up a VPS with a reverse proxy. Not the most approachable for most folks.
There's also relay.tools for setting up your own relay, which simplifies this, but I am not sure if that relay will import your existing notes.
I believe there is a tool out there for mass broadcasting notes to your current set of write relays, can't remember what it is. Without it, though, you would need to go to each individual note and manually rebroadcast them one by one.
