The procedure to implead the LRs is given in CPC in order 22 Rule 4, Rule 9 etc.
The plaintiff has to file a petition and affidvit with reasons to implead the LRs of the deceased defendant, it will be argued upon for its validity by both the sides including the party proposed to be impleaded, after which the court will take a decision either to allow the petition or to dismiss it on merits.
If the procedures have not been followed up properly the aggrieved person can file a revision petition before high court and seek to set aside the orders of the trial court which has not followed the rules in this regard.