A123Systems’ Hymotion has signed the first six dealers for its kits to convert Toyota Priuses into plug-in hybrids, the company announced in a blog posting Thursday.

Among the so-called Plug-In Pioneers are four Toyota dealerships, including Westboro Toyota in Boston, Fitzgerald Toyota in Washington, Toyota of Hollywood in Los Angeles, and the Minneapolis-based Denny Hecker Automotive Group, which sells multiple brands.

Felix Kramer, founder of CalCars.org, a plug-in hybrid advocate group, called the announcement “great news” and an important step toward bringing plug-in hybrids, which drivers can plug into electrical outlets for better mileage, to the mainstream.

For one thing, having plug-in hybrids offered by official dealerships will give Hymotion – and plug-in hybrids in general – far more credibility in the eyes of the general public, he said.

“This turns it into an official product that a Toyota dealer is supporting and gives it the dealer’s stamp of approval, which you wouldn’t ordinarily expect,” he said. “It’s really reflective of the change in the auto industry ­­­– consumers have a tremendous amount of clout and dealers have split loyalties. One would never imagine that the dealer would risk the relationship with the primary partner or the displeasure of its primary partner with this new relationship.”

The news is particularly interesting because aftermarket plug-in hybrid conversions detract from Toyota’s advertising message that hybrids don’t have to be plugged in, Kramer wrote in the CalCars.org newsletter Thursday.

“Toyota is concerned that any issues about quality or safety of third-party modifications could adversely affect the company’s image, or the prospect for ‘done right by experts’ [plug-in hybrids] made by Toyota,” he wrote. “But the interest levels are growing, not decreasing, and more than 150 conversions are on the road.”

“It’s still far more expensive than most people want to spend, so we’re still waiting for mass production,” he said. “But it’s one step closer.”

In April, when Hymotion began taking orders for its kits, the company quoted a price of $9,995, including installation, plus an extra $400 “destination” fee (see Can Hymotion Convert the Auto Industry?).

The company still wants customers to order the kits online, rather than from the dealers, but customers will need to have the kits installed by one of the certified dealers, Hymotion said.

The company claims its kits will enable drivers to get more than 100 miles per gallon by replacing some of the fuel with electricity, compared with the Prius’ normal 46 miles per gallon.

But Hymotion hasn’t said when the kits will be available, or in what quantity.

Regardless, Kramer said, the news puts more pressure on Toyota to develop its own plug-in hybrid.

“We hope this new development, along with GM’s announcement this week that the Chevy Volt will be scheduled for mass production in late 2010, will encourage Toyota to accelerate its current plans, which at the moment amount to a commitment to deliver 400 [plug-in hybrids] for fleet evaluation by 2010,” he wrote in the newsletter.