This is not supported unfortunately. Part of the reason is that loan and mortgage transactions are actually splits - one part with the interest and the other with the principal. When you download transactions, you get a single item - no split. This then is an issue on how you actually match up the transaction.
If you really need it, then consider the plus and minus side of changing the loan into a liability account. I'd personally not bother, but you might prefer to do so.
If you want the ability, then you'll need to lobby Microsoft. Try the
Keywords: mortgage, account, download, loan