Yes,
You can define your own custom exchange rates provider. For sample code please see here: https://gist.github.com/alex-georgiou/492196184f206002c864225180ca8fbb
You are advised to use code like the one above, rather than editing the database directly. The filters get executed by the plugin and the result of all the filters gets written out to the database.
Exchange rates are stored in the options DB table, the name is wallets_rates.
Hope this helps.
with regards