You can dink around insided the ShapeSheet to do this, but it's not super
straightforward.
Window > Show ShapeSheet will give you a spreadsheet that manages the
properties for a shape.
There's a LineWeight cell that controls the thickness of the line - this
changes when you set Format > Line or similar via the user interface.
The Geometry sections control the vectors for the shape.
You can enter Excel-style formulas in any cell, so you could create a shape
that has an inner or outer line-path. There's no UI setting for this
unfortunately.
For a rectangle with line hanging on the inside, the geometry formulas look
like this:
X Y
1. LineWeight/2 LineWeight/2
2. Width - LineWeight/2 Geometry1.Y1
3. Geometry1.X2 Height - LineWeight/2
....