I don't see any option for the import of clients. The single add option on the client page though specifies that it is for clients that have not connected.
So it might be easier to change the policy of the clients after they join, just a few clicks etc.
I was going to suggest doing this via API
PUT /networks/[id]/clients/[mac]/policy
{
"mac": "00:11:22:33:44:55",
"type": "Group policy",
"groupPolicyId": 101
}
However you'd have to most likely develop a python script to do this, and even you did, I believe Meraki limits the API. "Call volume is limited to 5 calls per second (per organization)"
So you would have to find a way to delay the calls, so it would only do a couple every second.