Of course it is. The game client itself already has most of the code needed to do that.
The client is made in a way to receive a lot of data and instructions about what to do from a server it is connected to. All the functions like - draw a sprite, draw a creature, move a creature somewhere, etc. already exist in the client, except it is waiting for a server to instruct it when to execute them.
If you cut out the server as a provider for these instructions and code in a way where all of this logic is supplied locally within files shipped with the client, you can make independent singleplayer content.