I did find out how to do what I want with the to_TYPE functions, but it is still a bit cumbersome because there is no way to save the functional formatting.
I found by reading the instructions that you can reformat the values on the export grid data page:
http://www.minq.se/products/dbvis/doc/main/doc/ug/exportPrint/exportPrint.html#mozTocId339765
So from my previous example, I could do:
to_date('$$value$$','YYYY-DDD"T"HH24:MI:SS')
I still think it would be great if there were a feature added to do this automatically. I do realize that the type formatting used for dbvis does not directly correspond to what can be used with Oracle (i.e. Oracle is not case sensitive, hence minutes in dbvis are 'mm' and 'MI' in Oracle). So translations would have to be performed differently for different databases.
At the very least it would be great to be able to save this as a setting, i.e. a default value string for a given type.
By the way, I think this is a great program!