Thanks for the welcome and answers. The plugin AnotherDXF2Shape works fine, but was too late. I asked the surveyors for a list of points in .txt-file. I transformed the coordinates from the local system into a world file and imported them into QGIS 3 as CSV. Then I connected the points in lines and polygons and saved the files as shapes. I lost some of data, but it was not crucial to me.