I would like to have a unique file name per user when doing import or export.

Short of doing a different export script line for each user... can the target filename and path be placed in a variable or field and then used during the import /export?

Can this be done?

Yes it can be done, and you pretty much described all there is to it:

1. define the variable

2. export using the variable name as the path.

Here's an example of this technique in a downloadable file.

http://www.filemakerpros.com/SAVEDYN.zip

Thanks for your ideas: I have been successful in applying this in my solution.

I have noticed that it works only with a variable: it won't work when I use a global field.

True, you can't use a global field, but if you want to store your path in a field, you can put it into a variable in your script and then use it.