Yes you can, on all versions of Caddie since Caddie 10. Copy the plotstyles into the plotstyles folder and Caddie will use them. To find out where the plotstyle folder is located, open Caddie then go to Settings -> Environment Settings and expand the Folders option.
Use the same technique if you get a plotstyle when you receive a drawing from a colleague that you are working with.
On Caddie 16 and later, you can get to the default plotstyles folder very easily by going to Utilitiess -> Browse -> Common Application Data Folder. In the list of folders you should see one called Plotstyles.