I would like to add some local roads to the scenery of my region which are not included in FSX Scenery.What is the best product to do that?Thank you

I'd suggest SBuilderX, by Luis Sa. There are tutorials available that outline it's use, nicely put together by Luis Feliz-Tirado.

Depends on where you are but Ultimate Terrain adds roads at different levels.

I tried to use Ground4k what is giving strange results.The local roads that I want to add do not seem to be in UltimateTerrain Europe .Thank you

If the roads are close to an airport using ADEX is a possible option, but SBuilderX is probably the best option. There is also a program FSX KML that is designed to use KML files drawn on/in Google Earth. All these solutions require the FSX SDK tools, in particular shp2bgl. The methodology is to place a positioned background image to use for tracing over, or if you have vector data (possibly from a gov't source or Open Street Maps) in shapefile or Surfer BLN format you can directly import those. Typically the default road textures (from terrain.cfg) are used, but if you have UTX installed, UTX creates new textures that you could also use (by simply pointing to the GUID UTX assigns in terrain.cfg). scott s..