Inserting data somewhere in a sequential file would require that the entire file be rewritten.  It is possible, however, to add data to the end of a file without rewriting the file.  Adding data to the end of an existing file is called appending.    
fout.open("filename.dat", ios::app)  //open file for appending
Program to append to the contents of a file
#include <iostream.h>
#include <fstream.h>
int main(void)
{
    string name, dummy;    
     int number, i, age;
     ofstream fout;
     cout<<"How many names do you want to add?";
     cin>>number;
     getline (cin,dummy);
     fout.open ("name_age.dat",ios::app);    // open file for appending
     assert (!fout.fail( ));     
     for(i=1, i<=number; i++)
     {
         cout<<"Enter the name:  ";
         getline(cin,name);
         cout<<"Enter age:  ";
         cin>>age;
         getline(cin,age);
         fout<<name<<endl;   //send to file
         fout<<age<<endl;
     }
     fout.close( );       //close file
     assert(!fout.fail( ));
     return 0;
}