How can I use the watchdog timer in interrupt mode?
The watchdog oscillator is very inaccurate, and if you look at the datasheet you'll see that its frequency varies between 111 and 121kHz depending on both temperature and supply voltage.

So while you could set WDIE and implement WDT_vect, you're much better off using a timer for this. Especially if you're using a crystal or external clock with more accuracy than the internal RC oscillator.