The prices from financial markets that can be pulled into Numbers using functions come from Yahoo Finance. You can bring in such prices, current or historical, to Numbers using the proper financial function only if such prices for a given financial instrument already exist in Yahoo Finance.
To check if historical prices exist in Yahoo Finance for a financial instrument, first go to the Yahoo Finance web site and punch in its name, code, or symbol in Yahoo Finance's search bar (CAD/RUB for example) and see if anything comes up. If it does, select the matching item in the list to come up and then check if there is data displayed in the historical data section for the instrument.
In case historical prices do not exist in Yahoo Finance for a currency pair you are interested in such as CAD/RUB, a workaround is to derive them from the respective USD pair for each currency assuming they both exist in Yahoo Finance along with historical data. For example, pull in USD/CAD and USD/RUB using CURRENCYH, and then divide the latter by the former to get CAD/RUB.
Note that the CURRENCYH function pulls in the historical market closings and only starting after midnight US time for the latest closing. You can fetch the latest quote with a bit of delay using the CURRENCY function.