How To modify an airsoft shotgun to shoot multiple bbs?

Yeah, never try to modify a gun unless you really know what you're doing. If anything were to happen to the gun, you can't get your money back as the warranty would be null and void. You can pump it 2 or more times to shoot multiple bb's but they are very inaccurate and reduce the velocity by a lot. If you really want a multishot shot gun, try buying a UTG Multishot. They're pretty cheap and work better than a makeshift mulitshot.

A 20ga shell most definitely could be loaded with airsoft bbs, it would not damage the gun, the bb's would not melt in the fraction of a second they would be in the gun once fired, and especially if they were in a proper shot cup. Would it be safe to try it? This is vague, do you mean safe to try shooting a shell loaded with airsoft bbs, or safe to try shooting it at another person? If you mean safe to try shooting from the gun, it should be. If you mean is it safe to try shooting it at another person, then no, it is not safe, the velocity would be way too high and at close range the shot cup could cause injury as well. Also, I do not think it is wise to bring a real firearm into airsoft, as what happens if accidently a standard shotshell ends up in the gun?

you can brake the magazine like that... you should just pump the gun 1time for 1 bb and 2times for 2 bbs and 3 times for 3 bbs etc. you understand? NEVER MODIFY YOUR GUN!!! it voids the warranty.