What I know and regularly use is not an Android downloader (does not exist I think) but the PC setup utility. On my windows PC I use Navigator Free for Windows and it's Setup Utility also for maps. This utility warns me, if an update do any of my installed maps is availabe. So I download the maps to my PC Navigator data directory and sync it from there with my Android devices (tablet and phone) using MyPhoneExplorer (for Windows and Android respectively).
You can't add OSM maps if they are not in MCA format, a proprietary format used by Mapfactor Navigator.
Using the Android version, there is the "Map manager" button in the app with which you can download maps.
If files are large, many, including Oldie and myself, prefer to download them through a Windows PC and transfer them to the Android device like Oldie explained. Apart from syncing the files, it's also possible to simply copy the mca files from PC to the Android devices, provided you locate the right Android folder.
This is a problem with this button. It looks I checked everything and can not find any button or function or option to add an extra map. When first lounched Navigator asks do I want to download maps. This means there is the Downloader inside but how to reach it next time? Once at least one map downloaded it does not offer this option again.