It was a little unclear from your question if you actually wanted to introduce wild specimens of the same species (xiphophorus helleri) to your swordtails or if you wanted to cross them with other swordtail species.
The reason (or one reason) why most keepers of rare livebearers dislike the idea of crossing them with other fish is that a lot of rare livebearers are either extinct or threatened in the wild, so people who keep them in captivity see themselves as the guardians of that particular species. It's like a zoo that has got hold of a herd of Przewalski's horses, they're not going to waste that material breeding them with New Forest Ponies. The problem with fish is that once hybridisation has taken place those fish are likely to get into the trade, where they will be bought unsuspectingly by livebearer breeders who think they are buying the genuine wild species and so end up ruining their own breeding stock. This is what has happened to endlers, very hard to get hold of genuine un-hybridised endlers these days. And endlers are thought to be extinct in the wild. So if all the endlers in captivity end up as guppy hybrids, the species will be gone.
As for getting wild helleri specimens and crossing them, presumably that is ok, but I don't know how easy it is to get hold of wild helleri. The wild guppies I have seen are such beautiful fish, there is no way I would go ruining them by breeding them wild shop fish. The best course of action, anyway, would be as Helter suggests, to get in touch with some expert breeders.
whenever i think about breeding using a wild swordtail, that analogy of a wolf mating with a malamute or husky comes to mind. 
/www.livebearers.org/
and if it did happen to mate with one of my females what would be the phenotype? maybe i should get a punnet square going
