You don't need any grease nipples! Take out the banjo bolt. Get an old fashioned 'lever type' grease gun. Unscrew the nozzle off the end of your grease gun hose, and screw the hose into the caliper gently. Its not quite the right thread, but if you're carefull, it'll be fine. STOP when it stops! Pump away! If that won't shift it, nothing will! If you need to get pistons out of the opposite caliper half, split the caliper, make blanks out of thick rubber sheet, stick it in the vice, nipping the rubber up over the transfer ports. (and use mole grips) if you need to (2 ports), pump away again. As your bleed nipples are paggered, you may have to free off the pistons in the banjo bolt side first, re-insert them, then make sure they're locked in the retracted position, hold them in this position whilst pumping again, and the opposite side will start to shift. You get the general idea. Never been beat using this method!
Of course, if you know you're going to need pistons out, you could always try the bike own hydraulics first. You never know, maybe they'll pop right out ... or not!!!