how to join two Navigation mesh? I have an existing navigation mesh and that works fine with steering actuator. Now there is a need to extend the current navigation mesh and join another navigation mesh of certain length.

Please have a look at the attached screenshot, in the first screenshot, as I have shown I tried using 'Object Join' but that just destroys my navigation mesh(2). The individual cells of the navigation mesh(2) merges to one big cell, which I dont want. 

As shown in the second screenshot, as I have shown, if I could merge the extreme cells of both the navigation mesh then may be the cells of each navigation mesh are retained. 

Please help me to achieve this. Thanks in advance !
You need to select both of the meshes and select join from the lefthand Tools->Edit->join area.

Once you have done that then you click on the physics tab on the right and select NavMesh Reset Index Values.

if you find you need to join two nav mesh's you probably need to go through and select all your objects, hit CTRL + A and select location, rotation, and scale for all your objects, then rerun the nav mesh generator, it should fix your issues and you won't need to join meshs.