Yes to both, you can overlay textures without affecting grid functions and you can add wall geometry on top of tiles marked as impassable. To read about how this would work in practise, check out Section 3.2 in the user guide: http://www.zkshao.com/downloads/TBStrategyGrids_UserGuide.pdf
There you can see how you would be able to define walls in the editor (define an InitialTerrain area with a blocking terrain type) or set these via blueprint (SetTerrain with a blocking terrain type). Hope that helps. 
Since you can set walls via blueprint, its also fairly easy to create a wall actor that would auto-detect which tiles it overlaps and flags them as impassable at run-time. The plugin has utility functions for that (world position to grid position conversion, for both square and hex grids).