Собстно как?

var
st, podst: string;
p: integer;
begin
st := 'the string';
podst := 'test';
p := 5;
st := copy(st, 1, p - 1) + podst + copy(st, p, length(st));
writeln(st);
end.
function myInsert(s, subs: string; p: integer): string;
var
i: integer;
st: string;
begin
st := '';
for i := 1 to p - 1 do st := st + s[i];
st := st + subs;
for i := p to length(s) do st := st + s[i];
myInsert := st;
end;
begin
writeln(myInsert('the string', 'test', 5));
end.