Democrats in Congress denounced a Reagan Administration agreement to increase textile imports from South Africa despite growing pressure to impose trade sanctions on that nation because of its apartheid policy. "It's outrageous," House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) said of the 4% boost, due to take effect Sept. 1. Sen. Edward M. Kennedy (D-Mass.) and Rep. William H. Gray (D-Pa.) also vowed to opposed the little-noticed agreement, arranged June 27. Kennedy said the move would be harmful to U.S. textile workers.