Like Greybird, I have the opposite problem and nearly always have to take fabric out of centre back. I've checked my 'Fit for Real People' book (my go-to for all pattern adjustments) and it suggests three methods for altering for a broad back. The descriptions are from the book.
If you're 'only slightly broad', redraw the armhole to get a little more width, up to 1/4 inch each side, giving half an inch overall.
If you're 'quite broad',cut and spread the entire back. The illustration shows the pattern cut vertically and spread 1/4 to 3/4 inch. This gives more room in the shoulders and waist area too.
If you're 'broad only in the upper back', the pattern is cut vertically from the shoulder to just below the armhole, then a slightly angled cut is made to the side seam. The whole armhole block is moved out and the side seam trued.
That all sounds more complicated when written down than it does when looking at the illustrations in the book, but I hope it helps a bit.