How can I combine high-level enchantments?

I assume you want to use a command to give yourself an item with an enchantment above 5. To do that, you can just specify all the enchantments you want in a list (marked by []), for example like this:

/give @p diamond_pickaxe 1 0 {ench:[{id:32,lvl:5},{id:33,lvl:1},{id:34,lvl:5}]}

If you want to modify an existing item, you have to throw it on the ground, use a /entitydata command and pick it up again. Example command:

/entitydata @e[type=item,c=1] {Item:{tag:{ench:[{id:32,lvl:5},{id:33,lvl:1},{id:34,lvl:5}]}}}

If you mean putting different high level enchantments onto the same item using the /enchant command, run the command twice, each time with a different enchantment and they will stack.

You can also do this without commands by combining tools/armor and an enchanted book in an anvil.