Level 53
Well, that depends if you want the easy way or the hard way. The easy way being just change the .css files to .scss and run mix. However, that could create some issues debt during updates if different team members are involved. The other way would be to either combine the css files and output to a .scss file which you can use in the sass() method. There are probably a few different ways to do it but it will work for you after the initial setup.