The only way I know of that works is to map a drive (e.g. using Windows
Explorer) to the Sharepoint folder, and access the source file via that
share.

e.g. if the file is in a folder at http://companyweb/General Documents then
I can map (say) Z: to

\\companyweb\General Documents

(I hope those are both readable at your end)

However, I do not know exactly what conditions have to exist for you to do
that. I suspect it relies on the fact that Sharepoint servers are
webdav-compliant and that recent versions of Windows (and/or Office) have
the necessary webdav client software to make this kind of connection
possible.

Peter Jamieson