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.

Composer window - save draft

2. Via Bulk Scheduler

Go to Publishing > Bulk Scheduler > Choose a bulk scheduling type > Edit your posts > Save Draft.

Bulk pulication - 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.

Content syndication - save 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.