The conversion would probably have been carried out by an individual, there are professional glass restorers who could do this sort of work, cutting the bottom off, or perhaps an amatuer with the right sort of equipment could have done it.
I agree completely with Pinkspoons, these are vases which have been converted.
I'm afraid we don't tend to discuss "values" here, they're far too subjective.
"Ikora" is the name of the range of glass made by the WMF factory. It's a wide range, with lots of different colourways.
But I doubt very much the conversion in to lamps was done there. They made lamps to be lamps.