"A player we have already agreed a fee for was in Cardiff yesterday and has agreed personal terms," said City chairman Peter Ridsdale.
"He will have a medical today and is then expected to sign."
It's thought Hudson, once promoted to the Premier League with Crystal Palace, will get a three year deal. He was skipper of Charlton last season as the were relegated from the Championship.
Cardiff have taken Maros Klimpl on a weeks trial. The Slovakian midfielder spent part of last season on loan at Motherwell.
"We have watched Maros Klimpl and he arrived in Cardiff yesterday," said Dave Jones. "He wasn't here in time to train, but will join in today and is with us for a week."
Aberdeen are also said to be interested and Plymouth as well. Klimpl is likely to be a free agent but he has to negotiate a release from his current employers in Denmark first.
Cardiff have still not ruled out a move for Paul Quinn from Motherwell. The full back could be the next player coming to South Wales.