if i convert a java file to kotlin, is there an easy way to git diff the kotlin file with the original java file? can i tell idea to diff a file with a different file?

You can select arbitrary files and even the clipboard, search "compare" in the action list (Ctrl+Shift+A)

yes but does it also work with git? (example: compare User.kt from my branch with User.java from master)

Likely not as Git won't see it as a move so you can't compare history. It would be a very specific command to git to compare 2 independent files. You could try selecting the added and deleted files in version control view and see if compare is on menu.