No. It is not supported to join Windows 2019 nodes into the 2012 R2 failover cluster. You can join there Windows 2016 but not the newer 2019.
Windows 2019 being added to an existing cluster expects the cluster to be running at the Windows 2016 functional level at least.
The newer 2019 version expects the cluster nodes to have self-signed certificates (ClusInfraCert) generated for their intra-cluster SChannel (TLS) communication on port TCP 3343. Which is not the case with the older 2012 R2 version nodes that only use Kerberos for node authentication and communication protection.
If you want to join the Windows 2019 into the 2012 R2 cluster you will get an error (after some timeout) stating simply that the 2019 cannot communicate with any node of the existing cluster.