A few months ago I read about a way to send fees for an existing transaction in a different one. I don't remember where I read that though. Is that already possible?
I'm not entirely sure on what you are asking, but based on the title, yes, it is possible. When you make a transaction, you can set it as an RBF (replace by fee) transaction which will allow you to increase the fee after the transaction is broadcast. However, this must be enabled first and can't be done to transactions that are not Opt In RBF.
It's also possible to indirectly increase the fees using CPFP (child pays for parent) where you spend the change of the first transaction with a fee to cover both transactions.
This is what I was talking about. Thanks a lot.
I am looking for a service where I can use those to raise fees of an existing transaction which may have not been sent by me. So the first option looks inviable: the sender needs to set it as a RBF transaction first.
I will look for information on the second option.