Plugin modification in wordpress

We are looking to extend the functionality of our real estate site that is currently using the Essential Real Estate plugin (https://wordpress.org/plugins/essential-real-estate/). Currently, the plugin allows the creation of purchasable packages that allow users to make listings, Included in these packages is an allotment of featured listings.
We would like to be able to add allotments of featured listings on for existing subscribers, so that they can keep buying featured listings separately from their package as an add on.