You can do this through regular expressions (regex) on an app like Sublime Text or Atom. Using Sublime for example, you can go to menu Find > Replace, put this regex on the “Find” field (?=\b\w+) and put the hashtag character on the Replace field. Click Replace All. Done!
One little thing, make sure the regex option is activated on the Find modal. It is a dot followed by an asterisk .*