When you add a new feed to dlvr.it, you select when you want dlvr.it to make the first post:
1) Post the most recent item now
2) Post the next time a new item is published
3) All items existing in feed now (up to maximum)
To post all items in your feed, select "All items existing in feed now (up to maximum)"
You'll also need to adjust the Maximum number of items to Post per update Cycle in the Feed Settings, which you can adjust after Feed Creation.
Set this to a number equal to or greater than the total items in your feed.
Then save. dlvr.it will post the items exiting in your feed up the maximums.
Keep in mind that some social networks have rate limits and may not allow you to post a large number of items at once. You can use dlvr.it's Trickle feature to space your posts out over time.