There is no issue of a transition.
In order from the headstock end would be the nut, cut nice and low for fretless, then the fretless area, then the fretted area of board that has been carved a frets height lower than the the previous part. The first fret would be butted up against the higher area of board, then as per a fretted bass for the rest of the neck.
When playing the fretless area, the frets are exactly the same height as the fretless board would be if it were there. When playing the fretted area the 'first' fret would have a fret behind it, butted against the fretless area, acting like the fret behind any other played fret. No transition to worry about.
Another way of visualising this is to forget about frets altogether. Get a normal fretless bass and at whichever end you want the fretted area, carve the area between the 'frets' away to the depth that you like your fretted string action, the raised areas left are your 'frets'
Its completely doable, but ultimately not worth it. 🙂