The implementation of NBT as is, for now, can't support this. We will add support for custom capabilities in the future and using them one will be able to make data storage that is persistent. This will most likely be added in 2.x.x branch, though.
By donating to developers you can speed up development, as with more resources, we can dedicate more time to MCreator.
It is a free project made by developers working on it in their free time.
The implementation of NBT as is, for now, can't support this. We will add support for custom capabilities in the future and using them one will be able to make data storage that is persistent. This will most likely be added in 2.x.x branch, though.