for example if i have created some static meshes and particles in UDK, is there any way to import this into UT editor. also can i import udk assets like stuff from DM-sandstorm into my UT4 or UE4 map??
Not directly, but you can export your assets as fbx/obj/png/jpg files and then you could import them into the UE4 editor. Otherwise there is no way to use those assets.