Alexandre -

I think that what you are looking for is a stroke that is inside the object boundary rather than centred on the object boundary. I believe that the former is not available in Inkscape and is not supported by the SVG standard. However, I think you can get the effect you want using Path > Inset, or Path > Dynamic Offset, or Path > Linked Offset, or Path > Path Effects + Offset. They provide different levels of control.

I'm not very familiar with these tools and don't know where the best documentation is. In playing with them I used various combinations of Edit > Copy & Paste In Place, Object > Raise & Lower, Path > Object to Path, and Object > Fill and Stroke.

- Robert