Dan,
It sounds like that what you want to create more a
concept map than a
mind map. Xmind is mainly aimed at the later type of maps, but they support also the first ones. You have to use what Xmind calls floating topics. Insert a central topic (Insert → Floating Central Topic) and put it wherever you want. You might want to prevent floating topics to snap in regular ones by checking Preferences → Advanced | Allow Free Positioning.
Hint:
Keep the first sheet of your map as kind of portal, i.e. it should only contain high level topics (a project name) with very short description (in the notes of the project name topic). Make a new sheet from the topic (Insert → New Sheet From Topic) and put project details on this sheet, so that every project has it own sheet.
You can also create links to other Xmind maps and put these links into topics, should your projects grow or you need to keep archive or...