Yes, non-voting cardinals MAY enter the conclave if they wish.
11 U.S. Cardinals took part in the conclave of 2013.
As many as they need, the seats are set up just for the Conclave, although I believe the number of voting cardinals is limited to 120 by the Pope's constitution.
There were 115 cardinal electors in the conclave that elected Pope Francis.
The cardinals use paper ballots to elect a pope. There are no voting machines used.
Three cardinals are chosen before voting begins in a conclave to be Scrutineers who oversee each vote.
Only those cardinals under the age of 80 who are able to travel to Rome elect the pope.
Only cardinals under the age of 80 at the time the conclave begins may vote for a pope.
His Holiness Benedict XVI was, like the majority of his predecessors, elected by the college of cardinals (voting-age cardinals) in a papal conclave in 2005.
He was selected by voting of the College of Cardinals in a secret conclave held in the Sistine Chapel of the Vatican. Voting is by paper ballots prepared by each cardinal as he votes.
They use a paper ballot and a pen. No modern voting machines are used.
See the link below for the Cardinals and the list.TOTAL CARDINAL ELECTORS: 81TOTAL CARDINALS: 199All figures correct as at Monday, November 26, 2012 [Except where Cardinals have recently died and have not yet been removed from the database]