I have had to create a macro to create a separate drawing No. as we are slowly transitioning from an autocad workflow to a solidworks workflow. As an interim I have made a macro that will create a drawing No. but still let me have the part/assy and drawing with the same name for ease of opening etc and remaining linked.
Macro in question: Drawing No. Macro - Pastebin.com
When I run it, it creates the custom property on everything open and I only need to run it on assemblies or top level parts.