You can mask everything except the finger (easiest way is to mask the finger then invert the mask). When you divide, only the unmasked area will divide. HOWEVER you will get triangles where the two different polygon densities meet. It’s impossible not to have that happen. (Try it on paper and you’ll see why.) The problem with triangles is that they divide differently when you further divide the mesh. Quads split into four quads and the surface is smoothed. Triangles split into three quads and the surface is not smoothed. So you will have to do some extra sculpting at the join to make it look good.
What I’d recommend would simply be to split that part of the model into a new SubTool. You can divide and sculpt as much as you want and no seams will appear.