Northwest Bancshares Inc. said Thursday that it would extend its stock repurchasing to another 5 million shares. The Warren-based parent company of Northwest Savings Bank (Nasdaq: NWBI) said Thursday that its board had approved the expansion of the program that represents about 5.2 percent of its outstanding shares. A previous share-repurchase authorization back in September 2011 involved 4.7 million shares, of which 2.6 million had been purchased. Northwest has repurchased 16.5 million shares…