Combined RSS feed for Multisite networks

Hello,

I have several blogs in my multi-site network and I want to send articles from all these blogs to only 1 Facebook page. However on Facebook’s instant articles configuration page, there is just one field for Production RSS Feed URL. It is not possible to input all RSS Feed URLs created for my each blog separately.

Is it any possibility to combine all RSS feeds into 1 feed?
For example
https://www.domain1.com/?feed=instant_articles
https://www.domain2.com/?feed=instant_articles
https://www.domain3.com/?feed=instant_articles
will be combined into 1 general RSS feed.

or anyone have an idea on some solution?

Thanks in advance.