I just asked myself the same question, I saw your post a while ago looking for an a way to do this and was disappointed that nobody had answered...
But finally I managed to find something which works with maps downloaded from
I found a detailed explanation of this here :
Read this carefully as they are several subleties (for example you have to use an older version of img2ozf, because the latest version produces files in format ozf4 which is not read by oruxmaps)
Hope this will work for you too !