??I'm not entirely sure of what type of add-in is being discussed here as the discussion has ranged from VBA to VB and dll files etc.??

If you're interested in VB and don't already have this, click the link below to download a free 'envelop' VB compiler that allows you to create your own stand-alone projects. (if that's what you mean)
As it also includes a packager, you can package the project, have links to XL (or the reverse) etc.

http://www.freebyte.com/programming/...s/envelop.html

Regards, John...