Minecraft – How to add an enchantment to another item that already has some enchantments

This may be possible using the Uncrafting Table added by the Twilight Forest mod, although I'm not sure if it works with regards to converting from one type of tool/weapon to a different type (i.e. pick to sword). The relevant excerpt from the wiki:

The Uncrafting Table also allows you to recraft items, which is often used to keep a rare enchantment on an item. The Uncrafting Table can also swap enchantments from one item to another. For example,an Ironwood Sword with Knockback 2 and Smite V can be recrafted into a diamond sword with more durability for the cost of 27 levels of experience. The experience levels are generated with the amount which the enchantment is roughly created at, meaning the rarest enchantments will cost the most experience levels.

You can enchant books instead, which always puts only a single enchantment on it, and then use the book in the anvil to apply it to a tool or weapon. You do have to go through a lot of books to get exactly the enchantments you're looking for, and it costs extra to transfer the enchant from the book to the tool on the anvil, but what you get for the loss of efficiency is precision in having exactly the enchants you want.

Note that you can also combine enchanted books of the exact same level on the anvil to get one book that has that enchantment level +1. So, you can take two Efficienty III books, combine them, and get one Efficiency IV book as the result. In this way you can slowly gather together the pieces for your ideal set of magic tools and, with enough XP lying around to go through this XP-inefficient process, get a set of tools to your precise specs.