With this method, you may access a decentralized exchange like Balancer or Uniswap and buy rETH using your token of choice, just like you should do almost every other token swap.
In Rocket Pool 1.0, the RPL token was https://dev.to/cleopatramorris_d539d19ea/rocket-pool-the-future-of-decentralized-ethereum-stakingintroduction-1m2b