It's depend on what you mean as "extending": official module/plugin or process memory patching. Some example would be helpful if you seriously want to discuss i

It's depend on what you mean as "extending": official module/plugin or process memory patching. Some example would be helpful if you seriously want to discuss it. Though for officially supported add ons system of proprietary products usually state what license you can or can't use. All kind of tools that access and patch process memory are not in danger because most of time they modify process state via 3rd party OS APIs and they do not interact with programs they patching. In same time changes that patcher do within process can't go under GPL and fact that developers keep licensing that code under open source licenses doesn't change the fact it's break EULA.