Create a dummy website in Server.app and specify a location you can access. Then edit the corresponding file using a text editor to point it to the real location i.e. /usr/local1/apache2/documents.
The file you need to edit would be in /Library/Server/Web/Config/apache2/sites/ and the file would have to places you need to edit. The first is labelled DocumentRoot and the second is labelled <Directory ".........">