WASHINGTON (Reuters) - Republican presidential candidate Herman Cain will propose an addition to his signature tax reform plan on Friday to help battered inner cities of the United States rebuild their economies.

The former Godfather's Pizza CEO, who has rocketed to the top of Republican polls, will announce in a speech in Detroit a plan to create "opportunity zones" in cities to foster small businesses and create jobs.

The proposal answers criticism that his 9-9-9 plan to limit personal and corporate income taxes to 9 percent and create a 9 percent national sales tax would hurt poor Americans.

"We carved out a substantial amount from the aggregate 9-9-9 Plan tax base, enough to exempt those in poverty, and we will work with Congress to best apply these in a way to break the "poverty trap" and replace it with positive incentives that encourage people to work and take risks in this economy," Cain said in a campaign news release ahead of the speech.

Cain has taken some heat for his plan because experts say it would increase taxes on middle-income Americans, mostly by introducing a national sales tax for goods and services, which has been deeply opposed by conservatives.

Fox News said Cain's opportunity zone plan risks angering unions because it would enact policies they consider bad policy, such as the elimination of the U.S. minimum wage.

(Reporting by Steve Holland; editing by Anthony Boadle)