Is it possible to use the built in microphone preamps as well as an external mic preamp coming into the unit at the same time?
Yes this is possible, however you can only ever get 8 channels of input at any one time, so for example you could have channels 1-4 using the internal mic preamps, with channels 5-8 using an external mic preamp connected via the DB25 analogue return (with direct access to the ADC).
The channels that you want to use for the external mic preamp just need to have the A-D button pressed on the front of the unit to break the signal flow between the preamp and converter and accept signal from the DB25 connection.
A benefit of working in this way is that there is no need for changing around the connections every time that you want to switch between the internal and external inputs. Both the XLR/Jack inputs and the DB25 inputs can be left in while they are not in use.