You will need to call the Windows API function ExtCreatePen() to create a geometric pen style, and use the Canvas.Handle property to use the Windows Path functions. Example:

procedure TForm .Button Click(Sender: TObject);
var
  lb : TLogBrush;
begin
  lb.lbStyle := BS_SOLID;
  lb.lbColor := RGB(255, 0, 0);
  lb.lbHatch := 0;
  Form .Canvas.Pen.Handle :=
    ExtCreatePen(PS_GEOMETRIC or
                 PS_INSIDEFRAME or
                 PS_ENDCAP_SQUARE or
                 PS_JOIN_BEVEL,
                 20,
                 lb,
                 0,
                 nil);
  BeginPath(Form .Canvas.Handle);
  Form .Canvas.MoveTo( 00, 30);
  Form .Canvas.LineTo(200,  00);
  Form .Canvas.LineTo(300, 30);
  EndPath(Form .Canvas.Handle);
  StrokePath(Form .Canvas.Handle);
  Form .Canvas.Pen.Width :=  ;
end;