While I don't have accounts with multiple currencies, I recall a post from someone that did. Basically select a "primary" currency and use that for your YNAB accounts. With a primary currency selected, you can then denominate your accounts that are in other currencies in your primary currency using today's exchange rate. I would suggest adding the currency for each account to the account name to help keep them straight.
When money moves from one account to another, calculate the correct values by converting to the primary currency. Periodically (maybe when a statement is received) enter an adjustment transaction to account for any changes in the exchange rate.
This is a work around, but is fairly simple and doesn't require a lot of additional effort. Hope it helps!
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