We are using MS Server Essentials 2012. There are already a few administrative webs it has for cert mgmt etc. The main one the users use is the one created when you setup Remote Access. I'd like to keep that functionality and add SP to it. But when I try to add the SP to an existing web, and when I select the Default web site, host headers (in the SP wizard) gets greyed out. Is it possible to make SP a sub site of an existing non-SP web?
It is possible, yes, but you really don't want to invite that kind of pain into your life, do you?
A better approach is to modify your existing port 80 default IIS web app so that it looks for the specific host headers for your existing applications rather than intercepting all port 80 requests. Then provision a new Sharepoint web application/site on port 80 using a new appPool and a new host header just for SharePoint. This will allow your existing apps to operate as usual and will still keep SharePoint as isolated as it needs to be.