You can tie a normal Somerville Bowline on a rope that's already attached to something -- you just use the working end of the rope as you normally would the center bight, and the end already attached to something as what would normally be the long end of the rope. The main limitation of this technique is that it doesn't work when you want tension between the old and new column ties; if there is already tension on the line, it's difficult to form and hold open the loop of the Somerville Bowline knot, and when the knot is tightened at the end, the tension will be lost.