Alimentation Couche-Tard Inc., the Canadian company that is attempting to take over Casey’s General Stores Inc., said today it has extended its $36.75 tender offer to Sept. 30. The offer, which has been rejected by Casey’s board of directors, was set to expire yesterday. Couche-Tard, which first announced in April that it was attempting to buy the Ankeny-based convenience store operator, said it has acquired 546,435 shares, or 1.1 percent, of Casey’s outstanding stock. Casey’s will purchase nearly 13.2 million shares, or nearly 26 percent, of its stock for $38 a share under a repurchase plan that was finalized last week.