Is it possible to use normal headphones for passengers in a light aircraft?
There are some solutions to convert a normal set of headphones into an aircraft headset, but in total they are going to cost you more than a base level headset would.  

One popular (available in TSO and non-TSO options) option is UFlyMike which is an adapter for Bose noise cancelling headphones that adds a microphone and connects to a standard 2 jack airplane audio panel.  But back to your issue at hand, this is not cheap.  The UFlyMike will set you back around \$200 and the Bose headphones are another \$300, making this adapted headset cost a total of \$500.  This is a bargain compared to an actual Bose aviation headset, which runs around \$1000, but is well above an entry level non-ANR headset.

Your best bet is to ebay some old clamp style headsets (e.g. DC) to get some cheap headsets and then keep around your current headset when you eventually upgrade (unless those trade-up deals from e.g. Lightspeed are too hard to resist) to build up a stock.  Alternatively, if you are renting, you may be able to rent a few headsets with the plane so everyone can be equipped for a trip.