From following link
https://learn.microsoft.com/en-us/azure/traffic-manager/traffic-manager-faqs#can-i-use-traffic-manager-with-endpoints-from-multiple-subscriptions
Where it says:
For other endpoint types, it is possible to use Traffic Manager with endpoints from more than one subscription. In Resource Manager, endpoints from any subscription can be added to Traffic Manager, as long as the person configuring the Traffic Manager profile has read access to the endpoint. These permissions can be granted using Azure Resource Manager role-based access control (RBAC).
It seems we can make it through RBAC.