The Fed can do this by launching their own bitcoin exchange and then dominating the mining of blocks on the blockchain (AKA a 51% attack). They can then prevent any other miners from being able to mine blocks, and they can configure their own miners to only accept transactions which are depositing bitcoin into the own exchange's wallet.
At this point nobody can sell bitcoin to anyone other than the Fed's exchange, at any price, because the transaction will not be recorded in the blockchain. The Fed can now set the exchange rate to whatever they want, and they can pay for the bitcoin they exchange with whatever instrument they want. They can run the money printer to buy your bitcoin with USD, they can make you accept US treasuries at a certain price, whatever makes the most sense for their balance sheet. Your alternative is to HODL and never be able to sell your coins.