Currently, you can configure all lab VMs to be created in an existing resource group instead of the lab creating a new RG per VM.
For ARM environments, every environment is spun up in its own RG to allow for better cost tracking user and policy management for resources created for that specific environment.
DevTest Labs now supports integration with Azure Shared Image Gallery (SIG).
For the scenario you outlined above,
You can extract the images for the VMs you would like to move to a lab
Upload these images to the shared image gallery
Attach the shared image gallery to your lab
Create lab VMs from these images
Upload images to a SIG: https://docs.microsoft.com/en-us/azure/virtual-machines/windows/shared-images-portal
Attach a SIG to a lab: https://docs.microsoft.com/en-us/azure/lab-services/configure-shared-image-gallery
Create a lab VM from a SIG image: https://docs.microsoft.com/en-us/azure/lab-services/add-vm-use-shared-image