var
tt:text;
l:string;
begin
...
assignfile(tt,'text.txt');
reset(tt);
while not eof (tt) do
readln (tt,l)
...
Closefile(tt);
...
assignfile(tt,'text.txt');
Append(tt); // This would set the position at the end of file.
writeln(tt,'blablabla');
...
Closefile(tt);
end;