Ronaldinho, 36, has been without a club since parting ways with Rio de Janeiro side Fluminense in September 2015, reports Xinhua news agency.
"We are trying (to sign him), even though it's difficult. We have been in talks for the past 20 days. We are trying to attract sponsors to make it feasible," Nacional President Jose Luis Rodriguez told Radio 1010 on Monday, January 9.
"Despite having played only a handful of friendly matches in the past 15 months, Ronaldinho can still contribute on the pitch.
"We are talking about a player who not that long ago was the best in the world," Rodriguez said.
"Apart from that, he won the Copa Libertadores (with Atletico Mineiro) only three years ago."
Ronaldinho was voted FIFA world player of the year in 2004 and 2005 and was a member of Brazil's 2002 World Cup-winning team.