You can't use an SD card as additional RAM. You can use it as additional storage space, but cards are not meant for, or capable of, becoming RAM.
Now, if you are wanting to make the SD Card "Internal" rather than "External" (which is what would allow for apps to be installed on it) then you would need to be running "Marshmallow" and also have a device that will allow for the change. Since you didn't mention what you have, I can't say for sure.
If you actually are running "Marshmallow", or can upgrade to it, then you would need a blank SD card to start with. Why? Because when you change the way the card is read (to internal instead of external) the card is formatted as well as encoded. This will erase anything that is already on the...