you could use something like this:
the only downside is, you will lose amp's volume controlling ability since digital side doesn't have that in the signal, so if you need volume control and your bluetooth unit has it... it is non-issue.
you could alternately use something else to send the audio through bluetooth (supposedly your bluetooth unit can receive audio through bluetooth)
in that case, something like this should work.
it should work in first case too, since it accepts analog audio through 3.5mm plug so all you'd need is rca to 3.5mm cable and then toslink from it to your bluetooth speaker.
Now.. that was the case if you read the actual message.
if you read the topic, it is other way around, in which case the latter toy should also work just fine.