Yes! It is possible to install the d!NK app without accessing the Windows Store. This procedure is called Sideloading.
Sideloading allows you to distribute directly to the users in your organization. For apps that you distribute through sideloading, you will be responsible for validating and signing them, because sideloading bypasses the validation and signing requirements of the Windows Store. Also, you are responsible for deploying any app updates to your users.
Please contact us at firstname.lastname@example.org if you want to request the implementation process for Sideloading your Windows app.