A mass rally is scheduled in Gaur, the district headquarters of Rautahat, to mark his entry into the TMDP.
He is alleged to have link with a bomb explosion in Kathmandu earlier last year.
Babban Singh, Sadrul Miya Haq to join TMDP after polls
"I would like to apologise especially to my brothers and sisters among the squatters," the minister said. "Even while watching the situation, I felt it was difficult for me."