Came across this recently when I had to post up a message to users’ stating some maintenance activity will be occurring over the weekend. Found a tool called Storefront messaging – will see it located near the end of the blog post below
https://www.citrix.com/blogs/2015/07/15/storefront-3-0-message-customization/
Article is a little old so if you are using Storefront 3.5 definitely need to vet it (mind you, worth vetting if on 3.0 regardless).