I've had good luck with two Sennheiser ENG mic systems (ME2 elements) into my Nikon D800 (AGC is switched off... meters show on the back screen) via a simple 2 XLRF->miniTRS adapter I built for about $8US. Input gain structure is crucial (into the transmitters) and likewise output gain from the transmitters into the D800.
Careful management yields audio (interviews) no worse than that acquired via Sony Z1 with XLR input capability, and far, far better than using the onboard mic. For anything other than interviews, I'll use an outboard device... Edirol R09HR (2 ch, usually fed by a Rode NT4 or a mix console REC OUT), Apogee Ensemble (8 ch), Mackie Onyx 1640 or PreSonus 24.4.2... direct to HDD, edit, mix, and sync the bounce in post.
Look at the Magic Lantern hacks that Mats suggested. If you can get the AGC off, then careful outboard audio into the onboard port could work, it seems to me.
HB