Second That.
The 2408 WILL take an analog preamp's line level signal and convert it to digital, sending it into the DM via either ADAT or TDIF. It has 8 analog inputs/outputs, but will connect 24 channels of TDIF or ADAT into your DM. it's a pretty useful device, but, if you don't already have a 2408, an add-in analog card is probably a better use of funds for 8 channels of A/D and D/A conversion.
I think Jarno's thought about the advantage of using an analog card (IF-AN/DM) stems from the recurrent opinion here that the digital to analog conversiion seems to be at it's best when using the originating device's clocking. Ie., when using the 2408, it's sampling clock will need to act either as a slave clock (from your DM) or master clock (to your DM) when converting the preamp's analog signal to digital. By using the add-on analog card, the DM uses it's own internal clock for all inputs, (M/Ls, assn returns, etc.) even those within the add-on card, resulting in (possibly, and very arguably) a better sounding analog to digital conversion.