You can build your own content library by saving them in drafts wherein you can modify and publish it on a later date.
There are three ways to save your content as drafts for them to become your content library:
1. Via Composer Window
Click Compose, type in your message, choose your target profiles and select Save Draft.
2. Via Bulk Scheduler
Go to Publishing > Bulk Scheduler > Choose a bulk scheduling type > Edit your posts > Save Draft.
3. Via Content Syndication
Go to Publishing > Content Syndication > Find Content > Select social profiles or blog sources > Personalize your syndicated content > Save posts as draft.
Once you have added your content, go to Publishing > Post Drafts and all your posts will be saved there which you can schedule at any time.