[1936 Ja 12, 17:2]   TOWNSEND BILL AS 'RIDER.' House Backers Will Seek to Tack It on Some Other Measure.     WASHINGTON, Jan. 11 (AP).—Congressional backers of the Townsend plan were said today by Representative McGroarty of California to have agreed to offer the program as an amendment to House legislation at the first opportunity.     Mr. McGroarty, who introduced the Townsend bill in Congress, said that about sixty of its supporters favored that strategy rather than an attempt to force the bill to the floor for a vote by petition, which would require the signatures of a majority of House members.     "We anticipate that changes in the Social Security Act may come before Congress this session, perhaps along lines of preserving private pension and unemployment insurance systems, and that would give us our chance," he declared.     After a conference today with Dr. F. E. Townsend, author of the plan, he said that he had been informed that an average of 250 new Townsend clubs were being organized weekly.     "It's a big movement—too big to be ignored," he added.