Running mechanical on di2 frame

[amazinmets733]

I'm contemplating purchasing a DI2 frame and running mechanical on it. The seller states:

"ELECTRIC OR MECHANICAL SHIFTING FOR MECHANICAL SHIFTING YOU WILL NEED A CABLE GUIDE UNDER THE BB."

Is this factual? If so how labor intensive and skilled is the job at hand? If it does work will shifting performance suffer?

 

[dabac]

Road bikes traditionally have the front shifter cable running down the down tube, changing direction at the BB shell and connect to a bottom-pull front shifter.
To keep the cable from wandering, and from eventually sawing into the BB shell, you need a BB cable guide. They look like this:https://www.bing.com/images/search?q=bottom+bracket+cable+guide&FORM=HDRSC2
Fitting one is neither rocket science or brain surgery.
If done about right, you should have a bike that shifts as well as a bike built for mechanical shifting from the start.

 

[BrianNystrom]

It would really help to know what the frame is. Without that information, all anyone here can do is guess.