Sadly NO.. The SD card will if formatted as internal provide extra storage and the system will take control where it places future installs and stored data and it encrypts the content so the card can not be viewed on other devices .. Format the SD as external will make it extra storage that can be removed and viewed on other equipment.
The issue you are experiencing is caused in the main by Internet speed and quality, and the source of the originating stream. many apps that offer free to view subscription content have such issues.. Do you subscribe to any of the stuttering stream content providers? There are several routes to go down to achieve potentially what it sounds like you need but perhaps a little more information on the where, when and what is causing the issue will lead to a better conversation in resolving your problems. I believe the Minix U-9H is a good product and will be able to get you to where you need to be with regard your media entertainment.
On the issue .. I use 128GB SanDisk cards in all my Minix U9-H, and that I find is more than enough for my needs.