Quite simply; no you can't. The preamps of the iD14 are wired up to the convertors and there is no way around this.
This functionality is available on the iD22 as it has fully balanced send and return points for each channel. As the iD14 is quite a bit smaller than the iD22, we couldn't fit these in.
If you wanted to use the iD14 with an external pre-amp we would recommend using the ADAT or S/PDIF via the digital inputs. If your external pre is analog only then you can go into the iD14 via the line inputs but this will pass through the Pre-amp circuitry. This may change the sound, for better or for worse but of course that is subjective to the individual.