With Swing in a JMenuItem I can set an accelerator to be KeyEvent.VK_PLUS with a KeyEvent.CTRL_DOWN_MASK modifier. If I do this, when I press CTRL+SHIFT+EQUALS, since the plus sign on a standard US keyboard is obtained by holding SHIFT while pressing the equals key. It works.

If in JavaFX I use an accelerator of new KeyCodeCombination(KeyCode.PLUS, KeyCombination.CTRL_DOWN), then pressing CTRL+SHIFT+EQUALS does not activate the accelerator. Nor does pressing CTRL+The_Numeric_Keypad_PLUS. Note that there is no KeyCode specifically for the numeric keypad plus sign as far as I can tell.