Most vendors will supply a netlists format document or a database structure document for the file itself. Most are in ASCII so not too hard to guess.
Depending on the tools to be imported or the target tools many programs are already available for this type of work.
Do you want to import many different types of netlist into one target application?
Or do you want to generate a netlist in many different formats from one application?
Or do you want all of the above 8)