On the odd (very odd!) occasion I've experimented with "fruit cider" for home-consumption only, I've picked & washed the fruit, & if the plums are large, I've cut them in half & removed the stones.
Then put the plums in a freezer bag in sized lots to suit, popped them in the freezer & gone back to milling & pressing the apples.
After the cider is fully fermenting & starting to calm a bit, I take the plums out of the freezer, & let them thaw until just starting to yield to hand pressure.
I then put the partially frozen plums into a very fine mesh bag (the sort brewers use?), tie the top securely & suspend the whole mesh bag in the fermenting barrel of cider.
I use a high breaking strain monofilament fishing line (easy to sterilise with a bit of SO2 solution or similar) to tie round the top of the mesh bag & suspend the plums in the cider.
Obviously being careful to ensure the other end of the nylon line is securely anchored outside the barrel...
Any thawed juice from the plums is put into the barrel as well.
After a couple of days - with very clean & sterile hands! - I lift the mesh bag out of the cider & while holding it over the barrel, squeeze the pulp with my hands until about dry, then lower the mesh bag back into the cider.
After a couple of more days I repeat the process (clean & sterile hands!).
The mesh bag will hold the skins, any coarse fibres & the stones if left in the fruit.
When I think everything of use has been extracted from the plums, the mesh bag is given a final squeeze dry & the contents of the bag discarded.
Note that plums can be very high in pectin so think about how you can overcome this; one way is to keep the proportion of plums low compared to the volume of cider. When I'm "playing", I use no more than 500g of plums to 60L of cider.
Using plums with well coloured skins (dark red / purple / black) can give a blush to the finished cider.
You may find you get a quite heavy deposit (lees) too.
Apologies for the long pedantic post - but this works for me. 🙂
Ray.
Nottingham UK