You could use WPAD on your network to point all clients to an i2p proxy. I’m not sure sure if it’s secure or private to be able to access both at once though.

function FindProxyForURL(url, host)
{
  if (shExpMatch(host, "*.i2p"))
  {
    return "SOCKS5 [proxy ip]:4447";
  }
  return "DIRECT";
}