OK, well here is how I do it now.
I have the MASTER dat open in Notepad++ (direct from CMP's dat folder)
I have another folder where I put the files I want to add called "Additions to Master"
The folder "Additions to Master" has already had a Dir2Dat made from it before.
So, as long as I don't exit CMP it will remember those settings/paths for me.
Those 'additions' go in the folder - I run Dir2Dat and outputted dat is saved as same name/location as before.
Additions.dat is already open in Notepad++
Notepad++ notices the changes and reloads 'Additions.dat'
I copy the contents of 'Additions.dat' and paste them onto the end of 'MASTER.DAT'
'Clear Cache' for 'MASTER.DAT' and quickly rebuild the additions into the master set..
This is why it would be useful to have "Edit DatFile" open with Notepad++
Yes, I know if I want a nicely alpha sorted dat, I just export from "Set Information" and CMP does that for me.
Thanks!
Oxyandy