I have a large amount selenium tests written in java but im trying to switch over to Katalon studio to use the different features they provide. Im finding this very laborious, is there a way to just copy and past functions and methods over??
As described here, you can use in-built Eclipse-like editor in Katalon Studio.
Just switch to </> Script tab near the bottom of your work window, and paste Java code. No need for a main() method, since this is a Groovy editor and your code in the Test Case is basically a method (in a Groovy Script class).