Firstly - welcome.
ASG do not produce the M15 series themselves - they have various OEM manufacturers including Cyma, G&P, Classic Army and Lonex
It's unlikely you will find something that will bolt-up onto the existing barrel nut, so you will have to remove it and replace with the one supplied with the rail of your choice.
For the most part the barrel threads are 'standard' - however this is airsoft so expect outliers and shitty tolerances.
Amazon sell various rails (now that Aliexpress have had a recent crackdown on Airsoft stuff) in keymod and M-lok varieties. They should - in theory - fit without too much trouble, just add a bit of grease on the threads to prevent galling and seizing.