If you receive a project from someone under an open-source license (like the GPL or MIT), then that license gives you the right to develop the project further and to add features to it that you think are useful.
Which feature you can add is completely independent of the features that the original author might be offering as an extended version of the project under a different license. The only thing that you have to look out for is that you don't copy code that you are not allowed to copy.
For example, the (closed-source) license of the extended version can contain a clause that forbids further distribution of the source code of the extended features.
But if you develop your features independently of the extended offering and without knowing the internal workings of the extended offering, then there is nothing stopping you.
When you want to distribute your extended project, the original license determines what licensing options you have for your own code. If the original is under the GPL license, then you will have to release your code also under the GPL license. If the original is under the MIT license, then you have more options for your code.
Something else to keep in mind is that the original authors might have gotten trademark protection on the name of their project. In that case, you would have to re-name your version when releasing it.