If you've already uploaded contacts to a list, but forgot to select Add to Autoresponders, you still add them after the fact.  

Create a CSV file which includes only column consisting of the email addresses to add to autoresponders. If you want to add all active contacts from a given list to autoresponders, then you can export all active contacts to create the list.
 

Re-import or upload the contacts to your database and on the Import Details step, select Add contacts to autoresponders (Fig 1). 

Fig 1

Finish going through the import process to add your contacts to active autoresponders.