An Army warrant officer admitted at the opening of his court-martial Monday that he was involved in a five-year conspiracy to provide classified military information to Warsaw Pact nations. Warrant Officer James W. Hall III, 30, of New York City, pleaded guilty to all 10 counts of espionage, attempted espionage and failing to obey Army regulations that were lodged against him after his arrest last December in Georgia. Col. Howard C. Eggers, the military judge presiding at the court-martial at Fort McNair, did not immediately accept Hall's guilty plea. Under Army regulations, he will hear the prosecution's case against Hall and defense witnesses before deciding Hall's guilt and punishment.