Hello Sourav,

Absolutely. B.Tech Manufacturing engineering students definitely have a scope to be able to get into DRDO and ISRO.

You can contact the HR department of your college to provide you with a letter of recommendation and you can send it to them for an internship as well. 

Usually DRDO and ISRO will be  easier to apply for if you complete your Masters as the competition gets too high in bachelors level.

Hope this helps,

Good luck!