I was wondering if it is possible to create some kind of custom addons store/provider/repository that integrates Regular Addon store.
The case scenario is that our customer has a buissnes system built into the episerver site and if it would be possible to update that system by a nuget feed like the regular episerver 7.5 updates and third-party addons it would be nice.
Yes you can. You can add your own feed to the Add-on Store in web.config.
<add name="MyAddOns" url="http://nuget.domain.com/nuget/" filterTag="AFilterIfYouWant" />