It's possible, but requires a good deal of technical knowledge and modding. You'd be better off just buying a DAB enabled head unit than modding the one you have (especially if you're not technically savvy). Just google "converting fm to dab" and you'll find plenty of technical articles on how to do it if you're up to it.
Also, is it really worth it? FM offers a more rich, full sound. And all DAB really offers is no hiss and marginally better reception in poorer weather. The reception gains aren't really worth it at the expense of sound quality in my opinion. That's probably why it hasn't caught on. DAB might as well just be called AM since it's the same concept: better reception but poorer sound quality.
Plus the radio stations need to be broadcasting via DAB (which I hear hasn't really caught on). You'd be much better off trying for an HDRadio head unit (though they're pretty pricey still).