I found a program that is able to extract custom fields from outlook and put them in a text or csv file. It has a moderate learning curve, but I was able to produce useable output in 30 minutes. Your results may vary.
www.ivitar.com/clipform
It has a free trial, fully functional but limited to 25 outlook items. Costs about $20. (I have no affiliation with them.)
I used the following template to put the items in a csv file that I could open in Excel. I included GTD custom fields "Action" and "Project". It handled them fine.
[Source=outlook:Tasks]
[Output=c:tasklist.csv]
"[Project]","[Action]","[Complete]","[Subject]","[Body]"
A few quirks...
...The field for notes is called "body" not "notes." This took me a while to figure out.
...It triggers the outlook security warning.
...It has not been updated since 2002 as far as I can tell.
Ken