This is already possible in Java Minecraft.
If you create your world with Game Mode: set to Creative then when you open Inventory you will be able to add nearly ANY Block or item into your inventory.
Also when placing blocks when in this mode you don't use any out of your inventory.
When you create a world in "Creative" mode cheats are set on so you can then change from Creative (Mode 1) back to Survival (Mode 0) by using the /Gamemode command.
So, if you are in Creative and want to go to normal play you would hit /g and then hit Tab.
/Gamemode will be now on your screen. Just hit the space bar, 0, and enter and you will be in Gamemode 0 (Survival).