Samarislowe -I recently added another ball of yarn to an already finished scarf, not because it wasnt long enough but because I just liked the wool I was using!!! I undid the bind off (quite difficult as the old wool was going 'piley/felty' and was kind of meshing together, and I knotted the end stitch to secure it!). I then put the loops (knit stitch) from the end stitches on the needle (my wooden Brittany size 19's) and then continued with the new ball of wool on the end of the cast on end. My scarf was pretty basic in its stitch formation/pattern, but I'm glad I took the time to undo the cast off etc, it looks really good and feels warmer now. I would recommend if you want to extend the scarf you have, and it wont affect the specific pattern you were working on (e.g. borders etc), then do it! I feel longer scarfs are generally better so that you can double wrap the scarf. My scarf length is taller than I am standing! But if I double wrap it, its level with the end of my top!