I believe it says on the Ernie Ball main site FAQ that they will not add a piezo to a model without piezo. It would be a case of new bridge, plus carving out some of the body to put in the electronics. I'd say sell your basic model and buy one with a piezo if thats what you want.
Yep ... I did it to my Morse (mostly because they don't offer one with Piezo and I LOVE that guitar) and Dan Desy added it to his Axis.
It's more expensive than ordering it from the factory. Buy the time you buy the saddles and preamp you'll be set back a good chunk of change. Add in the time to install or the hourly shop rate at your local repair shop and it's definitely not the cheapest route.
Do it if you want the learning experience. If you just want a piezo, buy one stock. Then it comes with a warrantee
I agree with beej that it's not a cheaper route on an EBMM, but it is a super cool project. On other guitars, it may be the only way to get a piezo, or it can be also be cheaper than factory installations.
in Poland, piezo option adds about 300 bucks to the price, so I think a better option is to just buy a new one with a piezo. I would cry like a baby if I messed up sth with my guitar when adding a piezo myself