Mod developer, please add the Costum recipes for AE2 ,for sieve and the compost ,from SkyFactory 2.5 to Ex Nihilo Omnia in 1.10.2.
or
please tell me how to add it to a modpack that i develope in 1.10.2.

You would probably use something like Crafttweaker to change recipes

That is for crafting, if you want sieve, barrel, compost or hammer recipes you would use my json system.

is it possible to do something like put something into a barrel of something else to get an item? (e.g. putting skystone dust into a barrel of lava for skystone)

No, I might add that in a future update, however.

I think that might be what gilyern was talking about originally

Okay, I wasn't sure

Well the sieve part you were on right track, but the compost he used the wrong word. But with skyfactory there was additional barrel input/output that were included. So that was what i thought when he said that. (I could always be overthinking what s/he originally said though)

So how to do it?

That what i originally meant, to add the ability to putting skystone dust into a barrel of lava for skystone, and from sieve dust to get skystone dust.
But until you done it can you tell me how to do it?
And another thing please add the ability while you working on the other thing to put sapling and leaves form forestry mod (or any mod with sapling and leaves) to make dirt?

The leaves thing is composting and there is a file in the config folder telling how to add it. You just need to get the ingame item name first

How to format file is told within configs. The item name may be found with crafttweaker if the tooltip command works or if another mod dumps the item names that are ingame. Not at a pc so i can't check configs.