I have accounts in banks from different countries (both supported by Tiller).
I would like to have my account balance in the other currency to be somehow translated to dollars based on an exchange rate that I would manually update over time so I can see my net worth trends in US dollars
You can use googlefinance to pull an exchange rate, and also have it by date.
For example this code will pull today’s exchange rate for USD:EUR, you can then use that in your calculation.