I think this issue needs some clearing up. As dwarfgourami says, they hybridise easily.
The main problem is not whether or not it can be done, but what you do with the offspring. If you leave them in your aquarium, or sell them as guppies, that's fine. Breeding some Endler blood back into our overbred fancy guppies will probably do them a world of good.
What isn't fine is selling them as Endlers. Unfortunately, that's done very often now, and many of the so-called Endler guppies sold commerically are in fact guppy-Endler crosses. This is especially true with female Endlers. It's just too tempting for retailers and wholesalers to sell mix-breed females as female Endlers and get two or three times the price per fish. Female Endlers and female guppy-Endler crosses are virtually identical, hence no-one but an expert on these fishes would know the difference.
In fact, some Endler breeders say that it is impossible to get real Endlers via retailers, and the only safe way is through clubs where the pedigrees of the fish are followed closely.
Cheers,
Neale