Configure Network Load Balancer across VPCs

Yes this is possible to be done. One of the blogs talks about capabilities of using IP address targets in target groups. These IP addresses does not necessarily have to be within the CIDR block of the load balancers VPC but can belong to any IPv4 address from the RFC 1918 ranges (10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16) or the RFC 6598 range (100.64.0.0/10) for targets located outside the load balancer’s VPC (this includes Peered VPC, EC2-Classic, and on-premises targets reachable over Direct Connect or VPN). https://aws.amazon.com/blogs/aws/new-application-load-balancing-via-ip-address-to-aws-on-premises-resources/

In addition to this lets say you have connected VPCs via Transit gateway, you can apply the above as long as you have attached the VPCs to the transit gateway and setup the routing tables.