Albert,
If you do have Workflow, instead of Script.ReturnValue, use:
Watch.SetJobInfo X, Value
Where X is the job info number(from 1 to 9) and "Value" is the value you want to assign, which is a string. You will run your script in Workflow before processing the form.
In your form, you can call &Watch.Jobinfos[x], where x is the number of the job info, from 1 to 9. This will allow you to use the value you've obtained from your script and stored in the job info.
This will work well if you have Workflow. However, if you don't have it, you won't be able to do it, and as Eric mentionned, there is currently no plans for execscriptfile to be changed to return a string.
Hope that helps!
Regards,
Rapha