You can silicone over the top in a pinch, but it’s not really ideal long-term. Silicone won’t bond well to the EasyJoint if there’s any dust or residue left, and it doesn’t have the same durability as proper grout, especially outside with temperature swings.
If the joints are super narrow and hard to clean out, you might try a resin or brush in jointing compound that can go over tighter gaps without needing everything cleared out.