Raz,
At the moment you presumably run the digital out to your receiver - have you tried the analogue out to the receiver and if so which sounds better?
If you are happy using the analogue out just reroute the front channels via the new pre-amp. You may have to tinker with volume settings but it is workable, particularly if your player has the facility to adjust the balance using a test tone. Your proposal to use a combination of analogue and digital may work in a similar fashion - providing the signals are available concurrently (like Inchinichi says) and they don't get out of sync.
Providing you don't try to connect both 'new amp' and receiver to the front speakers at the same time (

) I think you can try experimenting without risk of damage.
The other possibility which occurs to me is that if the Receiver has a tape connection you could connect from the tape out to the new pre. Volume control may still be hit and miss.
Jim