I don't think there's a built in (easy way) to send/export both the ticket information and the associated attachments externally (though if possible, I welcome corrections for this).

The best I'd recommend, copy the ticket info from the web gui and download the needed attachments from there as well, then import/enter into the 3rd party system.

It's a good enhancement idea, I suggest putting in the "Idea Exchange".

Another idea, would be to create users and create a development team in service desk for the developers to access and use.

I believe if you add another Team for the Partner you should be able to notify them through ZSD Ticketing.
Adding a Technician, Team, Partner Organization.

This is in theory and so far seems to work as listed below.

Example:

ZENworks Service desk is installed @ the company Digital Airlines
Henry is the Digital Air Technician
The 3rd Party Partner is "FixIT Company"
Angel Stanford (astanford) is our Partner FixIT Technician

ZSD Prep   

• Create a Partner Org - FixIT Company
• Create a (Partner) Team - FixIT Team

•     User Source
  • Create an account - Angel Stanford - acct astanford - email angel@fixit.com
  • Put Angel Stanford in the ZSD Partner Group    
•     ZSD LDAP Import so Angel is a "User" in ZSD
•     Configure Angel to have Processes
  
  • In ZSD Select Angel Stanford | User Information Operation Processes - Incident
•     Add Angel to the Partner Team called - FixIT Team
•     Verify email sent
  • [Setup] > User > Users - Select Angel Stanford | Information > Email (button)
  • Check [Setup] > Setup > Email > Log for this notice to go out.

Test

• Open an Incident request for a Digitalair technician assigned to Henry
• Henry needs to call the 3rd party for repair
• Henry reassigns the Incident by opening the ticket and selecting Team: "FixIT Team" (should be in the drop down)
• Henry sees the New Team and Technician Angel Stanford
• Email should be sent to angel@fixit.com

Angel works on tickets in queue as Partner (Alias email)

• Log into Technician Portal as astanford
• Selecting the REQUESTS > Incidents should display the requests
• Angel can do the necessary management of the ticket (close or pass back to another Incident team)

Optional email configuration

Sending email directly to Angel at FixIT w/Alias email account

If User Angel Stanford in the Digital Air User Directory with the Primary email as astanford@ditigalair.com you can configure an Alias and direct the Send To to the alias.

•     Create an account - Angel Stanford - acct astanford - email astanford@ditigalair.com
•     Configure Angel Standford an alias email which is Angels Partner email address  angel@fixit.com.
  • In ZSD Select Angel Stanford | User Information > Alias
  • Add angel@fixit.com
•     Configure Angel to receive emails from ZSD to the FixIT account
  • Angel User Information now has two email addresses in ZSD
  • In ZSD Select Angel Stanford | User Information > Send TO as the angel@fixit.com

As long as the Send To is the angel@fixit.com the test above should work.

Let me know if this works to your requirements.