Tutorial: Boundary Surface
Here we go!
Step 1: I have already gone ahead and made three projected curves for another surface. Once again, it is very important that the endpoints of your curves are coincident. If they are not then it will only make things more difficult.
Step 2: Select the boundary surface tool by going to Insert > Surface > Boundary Surface.
Step 3: Select any two opposing curves (I find that in this situation it is best to select the longer curves) for direction 1.
Step 4: Select the remaining curves for direction 2. This is where boundary surface becomes a very useful tool. You will notice options for each of the curves selected.
Step 5: Since we already have an adjacent surface we can make our new surface flow tangent into it. Select tangent to face on the drop bar.
Step 6: In this particular situation a message window shows up telling me that somethings wrong. There is an issue making both surfaces tangent to each other. Pressing okay will allow Solidworks to "propagate the discrepancy" and in my experience the output is usually, satisfactory. There are other options on the drop bar and its good to experiment with all of them.
Step 7: After pressing okay we get a nice clean boundary surface that flows well into the adjacent lofted surface.