YNAB does not have multiple currency support for different accounts within the same budget. There are two ways to deal with this:
For the user who earns and banks in one currency, but who often spends in one or more other currencies.
Pick a primary currency for YNAB and use that for all accounts.
When you enter a spending transaction in a secondary currency, you can use a currency conversion tool to enter that transaction's value in your primary currency. You can use the memo box to enter the actual transaction value if you’d like. In other words, all your work in YNAB is the primary currency.
When you reconcile the secondary currency account, get a currency conversion of the total bank balance and use that as the reconciliation figure. Use the memo notes you have made to mark the items as cleared and check they match your bank.
When all the transactions have been cleared, you will probably find that there is still an amount to be reconciled - this amount is the sum of all the fluctuations in the currency conversion rates. Post a single transaction to rectify and reconcile to zero, and post this transaction to a category called "Currency Conversions" or similar. Over time, you will be able to report on this category and see how much you have gained or lost from the fluctuations in currency conversion.
For the user whose use of multiple currencies is largely for separate purposes.
Create a budget for currency A and a budget for currency B. To move back and forth between the two budgets, go to file > open and select the budget you want.
If you move money between accounts, you simply reflect the true amount in each currency. For example, you move $100 from United States bank to your Canadian bank. There would be an outflow of $100 in the United States budget. Because of the exchange rate, it’d be an inflow of $94.00 in to the Canadian budget. No conversion is necessary, you just enter the transactions as they occur.
This approach is fairly straightforward, though it can be a little more work since you need to work in different budget files.