How to make an EFF file?

If I wanted to create an EFF file, which tool would I use to that end?

Tangentially, can an EFF file change the armor level code of an item?

Specifically, I would like to create an EFF file that checks whether the wearer of a robe is a Mage. If it is, then the armor code of the robe should be changed from xW to xV. If it isn't, the armor code should be set to xW. Is that something that EFF files can do?