Hello Dear Farzad,
If I understand your question correctly, your original signal is a 100 sec signal and you want to expand some part of this signal. First you can slice your original signal such as
interested_vector = original(50:0.5:55);
Then you should insert this part in the right place:
result = (original(0:50) interested_vector original(55:end));
I hope this gives you some idea to do what you want.