Importing third party VM's into VIRL is an option, however there is no official documentation. However, do check out the community generated content for notes on adding that specific image, which can be found here: Playing with Juniper vMX on VIRL
Please note: The following content is provided "as is." That page originally referenced an archived discussion on the old VIRL dev-innovate forum. Since that site is no longer available, the content of that discussion thread has been added into that page.
I know it's not much, but one alternative is to configure external communication for your simulation. This will allow your VIRL nodes to connect to an external network. For example, you would deploy your Juniper environment as a standalone system on a separate network (outside of VIRL) and have your VIRL simulation (configured for external communication) connect to the Juniper environment.
Please review the following guide: How To: Configure External Connectivity for Simulations (supplemental)
Cisco Learning Network.