A handgun in Cali must be on the roster - there's a couple exceptions to that, but for the sake of this thread I will not go into detail.
If a pistol smith owns a 1911 himself and customizes it, as long as it's on the roster, you can buy it from him and transfer it in without a problem. His customizations do not affect what the model name is from the manufacturer. For example: Pistolsmith owns a Springfield Pc9102 custom carry. He's customized it and you want to buy it. Since the 9102 is on the roster, you can buy it and transfer it in to Cali.
If it's of age to be curio and relic, and you have a c&r license, it's been customized...you can still transfer it in as it is still c&r by its manufacture date associated with the serial number.
If pistolsmith customizes a Colt series 70, you can not own it in Cali because it is not on the roster.
The gun must be on the roster, what the smith does or does not do has no effect on what is approved for sale via the roster.
Of course there is always the possibility that you own a handgun that is on the roster (or is not - depending on if you brought it in when you moved, etc). You send it off to pistolsmith to be customized, he works on it and sends it right back to you. This is by far the path of least resistance.