few questions before me girls come

put the feeder away at night to prevent rats or mice.
either mix it in or put it separate: it doesn't matter as long as they eat it!
keep them on their layers' mash for the start then introduce the treats/pellets after a couple of weeks.  It will be slow to start with as they'll find it hard to pick up lumps.  But they'll get used to it, you just have to keep trying!