Yes it is possible, but a lot of places are hesitant to flute a barrel if they don't know how the barrel was manufactured in the first place, there is a good chance the barrel will warp during the process.
Barrel makers generally flute barrels after they are stress relieved if that hasn't been done already in the manufacturing process.
Many barrel manufacturers will not flute another makers barrels.
Many smiths will offer barrel fluting services as well as some of the major gun companies……it’s just a matter of looking around and doing some pricing.
The price may make you cringe.
The amount of weight loss will be determined on how deep the flutes are cut and that info will vary depending on who does the cutting.