Existing sheep riding a new sheep

You can make a sheep riding another sheep by using the {Riding:{[Insert Mob Info]} NBT data.

If you want to simply add a sheep onto another, you can use:

/entitydata @e[type=Sheep] {Riding:{id:Sheep}}

The /entitydata command allows you to set the NBT data within a mob.
Its syntax is: /entitydata [entity] [new data]

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However, this does not work as stacked entities are considered two different entities, though it is the correct command.

Why?

When you use /summon with the {Riding:{id:[Mob Name]}} NBT data, the game automatically creates the mob for you. (All the other data, from UUID to statuses).

However, using the /entitydata command only appends on the NBT data you've added. So, adding on {Riding:{id:[MobName]}} isn't simply enough to define a mob.

Update: You can't apprehend an entity onto another entity without manually editing its entity. (Safeguard or bug).

Suggested way to do it:

1. Open your .mca world data with any NBT Editor such as NBTExplorer
2. Look for the location of the two mobs you want to merge.
   Use F3 to look for the chunk co-ordinates you're in. Don't use the raw co-ordinates.
3. Go to the chunk(s) that the mobs are in with your NBT Editor.
4. Go to the entity you want to be at the top of the stack.
   Entities are found in Chunk [X, Y] > Level > Entities > ...
5. Create a compound tag within the entity's compound tag. Name it Riding:
   The colon has to be there. Capitalization also matters.
6. Go to the entity that you want to be at the bottom of the stack.
7. Cut all the entries within the entity's compound tag and move it into the newly created Riding: compound tag you've created.
8. Now, delete the old entity's compound tag. (To clean up)
9. Save and open in Minecraft. Go to the location of the mob you haven't deleted.

If you want stacks bigger than 2, it gets more complicated.