You can enchant an item that is normally not enchantable with a couple of methods. One way is to use an anvil and an enchanted book in creative mode. In creative mode, the game ignores the normal rules of what can and cannot be enchanted.
You can also do it using commands with NBT data embedded within it. The format for the /give command is as follows:
/give [player] [item] [quantity] [data value] [NBT tags]
For this, we're interested in the [NBT tags] part. The format for the enchantment tag is {ench:[{id:x1,lvl:y1},{id:x2,lvl:y2},...]}. So the full command for just giving a player an enchanted item would be as follows:
/give [player] [item] [quantity] [data value (normally 0)] {ench:[{id:x1,lvl:y1},{id:x2,lvl:y2},...]}
Check out this post on the Minecraft Forums for more details about using NBT data in commands.
For example, if I wanted to give myself (or the player) a stick with Sharpness V and Silk Touch I, I'd use the following command:
/give @p stick 1 0 {ench:[{id:16,lvl:5},{id:33,lvl:1}]}
You can do this for any item, any enchantment, and any value of level.
Also, a note: using the /enchant command does not work. The /enchant command respects all normal enchantment rules, regardless of the gamemode of the player. Even though you can make an Efficiency V stick with an anvil in creative mode, you cannot do it using the /enchant command.
Basically, just enchant the item again with the already enchanted item on creative. You can't double enchant items in survival as far as I know, so this is the command;
/enchant (your username) (enchantment) (level)
Just do that twice, and you should be able to have two enchantments at once! (tested on 1.9)
Best Answer
Ok, after 4 hours of commandblocking, I got this:
Now to explain:
to activate this all, just use this command:
The CB #1 is to replace the redstone block with wool:
The CB #2's make new scoreboard objectives:
The #3's have this:
The CB's labeled 4 use this commands:
Now, the line tagged #5 has:
Finally, those with #6:
The 7-15 commands vary only in the items, enchant id's and scoreboard stuff:
7 - iron_helmet, 0, selectMe
8 - iron_chestplate, 0, selectMe2
9 - iron_leggings, 0, selectMe3
10 - iron_boots, 0, selectMe4
11 - iron_sword, 16, selectMe5
12 - bow, 48, selectMe6
13 - iron_shovel, 32, selectMe7
14 - iron_axe, 32, selectMe8
15 - iron_pickaxe, 32, selectMe9
This system will obvioulsy work only with certian (iron) items, will give only 1 enchant and will need for the Player to hold the item whilst entering the command - see higher.
So you would need to use a giant system for auto-executing a enchant, but if you just use the give command:
Hope helped a bit.