1) I haven't tried it, but this should work. If you export the .db from gpxview, and then load it into Maemo Mapper like your doing, go into the Browse POI menu option and export all your selected POI's from your .db file to a .gpx file. Then, switch the Maemo Mapper POI database back to your default, choose Import POI and choose the .gpx file you just saved. Kind of roundabout, but it should work. I was going to do that, but I just manually added my geocache poi's. I may still try this method, depends which one is faster.

2) This is how you manually add any POI, whether it's an address, geocache, gas station etc.

Click and hold on the map with your finger (or stylus for more precise selection). Go to Tap Point > Add POI....
This will pop up a dialog to add the POI. The Lat and Long coords will already be set by where you clicked and held, you can then give the POI a name, select the category (which will help with either an icon or to find it easier) and you can write a description.
When you tap on the POI after its created, the info box will show the name and the category. To view the description, you'll have to tap and hold on the POI, choose POI > View/Edit...