I have the same issue as you - I'm using Google XML sitemaps but wish to edit my robot's text file.
I noticed the XML sitemap plugin has the following checkbox:
Add sitemap URL to the virtual robots.txt file.
The virtual robots.txt generated by WordPress is used. A real robots.txt file must NOT exist in the blog directory!
I think that the line 'a real robots.txt file must NOT exist...' only means it must not exist if you want the plugin to add the sitemap url to the virtual robots text, as it has to write to this file.
I also think that if you copy the contents of the virtual file and paste it into a real robots file, you should be able to then upload and edit it as you wish (as long as you untick the checkbox mentioned above).
I don't think there is any way to use the plugin to actually edit the robots text file.
I have to weigh up 2 choices:
- either have a virtual robots file which I know works, with the correct sitemap url but have the search engines pick up admin pages that I don't want indexed
- create and upload my own robots file and cross my fingers that it does not clash with the virtual robots file generated by Google XML Sitemap plugin / WordPress.
I don't actually know if this will work and like you, I've been searching for the answer - but I'm going to create my own file this weekend and I'll post back here if it was successful (or not).
At least your current robots.txt file isn't blocking anything.
One more thing - you mentioned All in one SEO pack - I use this on all my sites as it's an extremely useful plugin and does not clash with any other plugin I've got installed.