Hola Richard

It's not always possible to add por when using excepto. The use of the preposition is mostly possible if the element that comes after excepto has somehow a "causal" meaning. For example, you can say:

Me gustó mucho la fiesta excepto por la banda de música, que era muy mala.

I liked the party a lot except for the music band, which was awful. 

Meaning "except the fact that the band was bad, that was the reason why I didn't like it." 

So, it's not really interchangeable all the time, although that may be the case in English. 

Saludos