Last night I sent a transaction with the wrong fee (almost 0 BTC).
There's a ton of transactions right now and 24 hours later it's still not confirmed.
Is there a way to add coins to the transaction fee after sending it to speed things up?
Nothing , not even 1000 Satoshi ? If that's the case then it will probably never get confirmed and no it's impossible to add coins to the transaction after sending .
All you have to do now is wait , the BTC should come back to you (most likely 5 days or something)