So basically, you can put like $3.00 in the payment field of a transaction in the ledger and press CTRL-Q to bring up the currency list and then for example select US Dollar and click USE and the $3.00 CAD (which is the home currency) is converted into US Dollars.
Unfortunately, this method does not work within split items - so if you have other items in the split that are also US Dollar, you would have to convert them elsewhere and then input the amounts in the split.
