Is to make a selection the size of the box and then go to Select/Modify/Smooth. You can round the shape up to 16 pixels. If you want to round the corners more repeat the step until you see the desired effect.
Now that you have the rounded corners go to Edit/Stroke. In the pop-up window you can select the Stroke width, whether you want the stroke inside the selection, outside it, or centered, and various blending options. Remembe r the stroke thickness you use, because you'll want to match it if you subsequently divide the rounded box into sub-sections. Now click OK. You should have a rounded box with a thin outline around it.
Now use the line tool to divide the box into sections if you want to do that. Set the line tool weight to match the stroke you used above.
Is that what you meant?