Hi Paul & Ruth
If you are only adding 1 or 2 12v lights, then you should be OK to connect them to the existing wiring in the way that you've stated. You could also use spade connectors or connector blocks. You won't need any extra fuses, but ensure that you use a 12v supply that's already being used for the existing lights, i.e. don't tap into the fridge 12v supply for example.
If you have a new 'van then it may be worth checking for existing wiring. As with cars, the manufacturers sometime fit wiring looms that will cover all models, and it makes it easier for dealers to upgrade later. So just look around if it's in a place where lights are fitted on a different model.
It would be useful to have a voltmeter just to test the supply and make it easier to diagnose any faults that may occur. For example, I've fitted new lights before only to find that one wasn't working. It turned out to be a duff bulb, but a quick check with the volt meter confirmed that the power supply to the bulb socket was fine.
Also ensure that you cover up any exposed connections and allow space above and around the light for heat dissipation.
If in doubt though, get a competent electrician to do the job. Even a 12v system could cause a fire if it overheats or shorts out.