You need, as mentioned, the focal manipulator to add enchants to thaumcrafts wand focii.
You can add 5 enchants to a focus, the first costs 8 levels and each subsequent enchant 8 more (up to 40 for the 5th enchant).
An additional wrinkle is that certain focus enchants are only available at certain focus levels. The excavation focus accepts enchants as documented here
http://farproc.wikidot.com/thaumcraft-ranges.
Basically you can add Treasure (Fortune) at ranks 1, 3 and 5, Enlarge at ranks 2 and 4, Dowsing at rank 3 and Silk Touch at 5 (Obviously don't add Treasure if you want to take Silk Touch eventually).
Enlarge II - the max level on the excavation focus, will give you a 9m diameter circle, but a rather useless radius on a hand wielded focus, never add Enlarge to a focus you want to use in your wand.