It is possible to combine hub and derailieur gear systems on the same bike. The Sram 3x8 and 3x9 systems use a three speed hub with multiple sprockets and are found on several folding bikes. This system works well and is worth considering.
 
It is also possible to fit two chainrings to a hub geared bike, making a three speed into a six speed. This is only worth doing if you are very careful in choosing ratios, as it is easy to end up with only one extra gear.
 
Some current Bromptons have two sprockets on their three speed hubs.
 

 
The picture above shows two Shimano type sprockets fitted to an AW in a Dawes Kingpin.
 
These composite systems have never been all that popular, and we very seldom fit them. It is better to just buy a five or seven speed hub than make something work from mismatched bits.