well if you have a good quality fibreglass part, it is possible to bond it on to steel, but not if you use polyester, you need to use epoxy, good e-glass rowing or bi-directional glass.BUT the steel has to be 110% clean, i used a 60 grit sanding wheel on an angle grinder, and then cleaned with first spray on clutch and brake cleaner, and then a silicone remover on a lint free cloth. that way the fender extension is glassed onto a large area of clean steel, useing a quality epoxy (it wont fail before the steel does- trust me)
but you HAVE to buy the epoxy, and it has to be one that is specified to bond to steel. i cant stress this point enough, polyester resin just doesnt wet the steel properly, i Contacted the local West-epoxy dealer. I am not sure where you would find one in your area, biut try a web search.