to_date('270996','ddmmyy')
Or you can use substr to break the dd, mm, yy into individual components and then use concat to concatenate them in the desired format . At the end use the to_date command to put it into date format.
a = 270996 (of char type)
dd = substr(a,1,2);
mm = subst(3,2);
yy = substr(5,2);
complete_date = dd||'-'||mm||'-'||yy ;
date_format = to_date(complete_date, 'dd-mm-yy');