Gluing copies of a 3-dimensional polyhedron to obtain a closed nonpositively curved pseud omanifold

Let S be a smooth 3-dimensional nonpositively curved Riemannian manifold with corners, whose boundary consists of a finite number of geodesically convex nonpositively curved faces (for example, a Euclidean or hyperbolic polyhedron). We show that it is always possible to glue together finitely many copies of S so as to get a nonpositively curved pseudomanifold without boundary.