Capacity of the Lafayette factory will increase by at least 100,000 units by 2016, the source said.
The Impreza now is produced only in Gunma, Japan.
Japan's Nikkei business daily reported this year that Subaru is planning to invest $230 million to increase the Indiana plant's output.
Subaru's parent company, Fuji Heavy Industries Ltd., has said the extra capacity is needed to end a shortage of vehicles at U.S. dealerships.
The United States is Subaru's biggest market and accounts for 50 percent of the brand's worldwide sales.
Subaru has been redesigning its lineup to better suit U.S. tastes by increasing interior roominess and adding high technology features.
The Impreza likely will be built on a second assembly line, according to the source. The factory produces the Legacy sedan, and Outback and Tribeca crossovers, on one line.
Last year the plant produced 184,783 Subarus, up from 161,716 in 2011.
Subaru hasn't decided whether to end production of the slow-selling Tribeca.
The Lafayette factory also produces Camry sedans on a separate line for Toyota, which owns 16.5 percent of Fuji.
Last year the plant produced 93,126 Camrys. Toyota spokesman Mike Goss said there will be "no changes" to Toyota production at the Lafayette plant.