You can import your already published posts to OnlySocial. You can then duplicate these posts and have them rescheduled to all of your platforms.
Video Guide
How to Import Posts
Go to the Posts Page from the main menu
Click on the Import Posts button at the top-right
Select the number of posts that you'd like to import
Select the social media accounts that you're importing from, and click the Import button.
You'll now see all of the imported posts at the top of your Posts, you can then duplicate, and edit to republish such posts.