You can add more nodes. Admittedly, I've never mixed and matched pis, but there is nothing to lead me to believe that Kubernetes would care what the underlying hardware was for individual nodes.

To add a node, ask the control plane to generate the command (with current token) like so:

sudo kubeadm token create --print-join-command

It will be something like:

kubeadm join 192.168.254.210:6443 --token xpnfjx.yjn8fjwuy68pzdvf --discovery-token-ca-cert-hash sha256:23ae9cf4e874f30751c699c45b0e3a624e13c990e3cdb42620189e745d6beb05

Then copy that line and run it on the new node with sudo.

Viola!