What are the dimensions of the stock muffler and the aftermarket muffler you might use? What is the volume?
I have added and extra emergency tank on my wifes XT225. I fabricated it with the intension of making the bike look had it had dual exhaust. I fabricated a 2" oval to look similar to a FMF muffler. if the volume is close to what you are looking to add with the muffler volume it is a very simple fabrication out of sheet alumminum or pipe.
I made this out on stainless since the pipe shop has the material handy.
I thought I could purchase the end cap from FMF to make it look right but found out I could not. I was hopping that I could run a fuel line to the carb, but the tank seemed too low. This gives about a 1/2 gallon extra fuel which extends the range about 30 miles.
If I remember correctly (don't have my pipe chart handy) 4" pipe about a ft long should give you about 1 1/2 gallons. I need to check, but it sounds about right. 4" pipe has 4 times the volume of 2" pipe.
For what it's worth. And as I look at this post. Way more information then you need.