Not directly, but going through a gateway.
All external currencies in Ripple (fiat or crypto) are actually stored at gateways, the ripple ledger only stores who controls which amounts of these.
For example, to send BTC to Ripple via Bitstamp, you need a Bitstamp account where you send the BTC first. After that, you can withdraw the BTC to the Ripple, and use the BTC.Bitstamp within Ripple to trade against other currencies.
Alternatively, you can send BTC using dividendrippler.com which is a gateway that only holds crypto currencies.
Or you can deposit cash using zipzap and exchange that into BTC or whatever currency you want.
To withdraw money from Ripple, you need to go the reverse way. For BTC, there's a payment bridge operated by bitstamp which you can use to send BTC to any BTC wallet address.
Onkel Paul