The simple answer: no.
The deeply nuanced answer: NO!
The concept of abandonware has absolutely no legal basis. It is basically an idea that a bunch of people came up with to legitimise software piracy for software that is no longer published since (a) the original developers and publishers no longer use the software as a revenue source; ( B) there are many cases where noone knows who owns the rights to the software anyway, so it cannot be released legally; and © because very few publishers or developers care enough to prosecute.
Regardless of what your stance on abandonware is (and it ranges from pirating obsolete software is harmless to any form of piracy is illegal) it is illegal. You should be aware that such software is not in the public domain, so you cannot simply base other products off of it. If people were to start doing that, publishers and developers would probably start caring a whole lot more about people illicitly copying obsolete software -- and would probably be liable to crack down on any use of their obsolete software.
If you want to modify the software at will, you will have to wait for the copyright to expire. At that point, it will enter the public domain and you will be able to do what you please with it.