Right now you're running navit in its most basic configuration. There are plenty of fun tweaks that can be added if you have the patience.
You can have multiple maps but only one mapset. It would look something like this:
<map type="binfile" enabled="yes" data="/usr/share/navit/maps/osm.bin"/>
<map type="binfile" enabled="yes" data="/usr/share/navit/maps/osm2.bin"/>
<map type="binfile" enabled="yes" data="/usr/share/navit/maps/osm3.bin"/>
If the maps exist and are enabled as above, you go into Settings -> Maps to check/uncheck the ones you need to use.
For the zoom levels from maps that you grabbed from OpenStreetMap, they are already sufficiently detailed. There's a keyboard shortcut for zooming but I can't find it or it's not working anymore. I think it was an F-key. Otherwise you need a mousewheel to zoom which ain't so great with our TB's. However there are plenty of On Screen Display (OSD) tweaks you can do to help:
Look for these two lines and enable them (change "no" to "yes") to get two buttons on screen where you can tap to zoom.
<osd enabled="no" type="button" x="-96" y="-96" command="zoom_in()" src="zoom_in.png"/>
<osd enabled="no" type="button" x="0" y="-96" command="zoom_out()" src="zoom_out.png"/>
Where it gets cool is you can copy a bunch of osd tweaks from here for some pretty cool layouts. Sometimes I use them for insipiration by picking and choosing individual osd lines from some of their layouts instead of using their entire kits. http://wiki.navit-project.org/index.php/OSD_Layouts