If vanilla Minecraft cheats (which allow the /gamemode, /give, etc. commands, as opposed to NEI cheats, which I explained above) are enabled in the world (which is probably not the case, but might be- worth a shot), you can type "/gamemode creative yourUserName" to put yourself in Creative mode, which grants you free flying, invincibility, and replaces your normal inventory screen with an item panel similar to NEI, but which I find more difficult to use. You could instead find the internal ID of the redwood sapling and use the /give command to spawn it in, which is probably simpler. The ID should be the number shown at the end of the first line of the text shown when you hover your mouse over the item in NEI. In my FTB Infinity instance, for example, it's 499, so by typing "/give Someone_Else_37 499 64", I can spawn in a stack of the saplings.
I don't know if the Ex Nihilo configs allow you to add arbitrary drops from the sieve. However, if they can, you'd need to add them to <your FTB install directory>/AgrarianSkies/config/ExNihilo.cfg. I don't have the mod installed anywhere handy on my computer, so I don't know exactly what to look for, but if there's anything there that says to list items for the sieve to produce, that's where you'd add the saplings. If there's nothing useful in that file, look in ExAliquo.cfg or any other Ex____.cfg file for the same thing (in case there's any ExNihilo addons in AgS I'm forgetting). Again, I don't know if any such option exists... does anyone else here happen to know?
If that doesn't work, you could improvise by using the sieve to produce a few stacks of spruce saplings, say, throw them off the island, and use that to justify spawning in some redwood saplings. I figure if redwood saplings were obtainable through the sieve, they'd probably be significantly rarer than vanilla saplings (even though you need a lot more of them) because Jadedcat is mean (No offense intended, Jaded.), so you'd more or less be replicating what would happen if the sieve actually did give them.
