Hi,
I'm a software engineer and PHP expert, I can write the cron script for you :
I will need to alter your database (add a new table or a column to your already existing table that contains the RSS feeds list).
The current code base will not be affected as the cron script will be independent and only uses the same database as your website.
I'm familiar with slim framework, so for curiosity purposes do you have a web service associated with the web site? otherwise why is it written in slim framework ? it's really not suitable for web site ...
Cordially, Mawen