Matter does not have to cross existing horizon for horizon to grow. Imagine a horizon radius R for a mass M (R=horizon(M)). Despite no matter would be seen touching that, time in sphere between R and R+dR ticks slowly even for external observers. So matter can get into this thin sphere in finite time. As soon as dM of matter gets into this sphere so that horizon(M+dm) > R+dR, the black hole now has increased radius and mass. Everything that was frozen on the surface of BH is now inside the BH.
