A direct import of OVA files as EC2 is not possible. You must first convert it to AMI. However, this process is not supported by VMware for the vSAN witness appliance.
With VMware Cloud on AWS, you have a fully functional vSphere environment in AWS with ESXi hosts and you can import OVA files directly as in any other vSphere infrastructure. So, this is supported as long as you meet the witness networking requirements (latency, throughput, etc).
See:
VMware Cloud on AWS
VMware Cloud