Adoptable Storage is a feature introduced with Android 6.0 (Marshmallow). It allows a phone to effectively turn a microSD card into its own internal storage. Some brands still allow Adoptable Storage in their stock ROMs, but Samsung is not one of them. However, it now seems that it is coming back in future updates that involve Pie (9.0).
https://www.notebookcheck.net/Android-s-Adoptable-Storage-feature-may-be-coming-back-to-Samsung-devices.361381.0.html