These stickers are png images 150x150 32bpp (obviously with Alpha-channel/transparency).
I based mine in the same files as the stickers I downloaded form Samsung trying to achieve maximum compatibility.
I put them in the /data/data/com.android.providers.calendar/sticker where the originals reside and changed (using ES explorer/root on) the permissions to be the same as the originals -rwxr-xr-x plus the owner and the group to u0_a151
But unfortunately the extra stickers aren't appearing in S-planner even after Reboot.
So. There must be something else to consider.
Perhaps another property of some kind, or there is a cache with all these icons that didn't update, or maybe someking of a log where I must add the names of the newest stickers, that I named them in the same fashion as the originals.