To import an openssl based generated private key and certificate into java keystore, follow the instructions below.

First you will have to create a new text file, which contains the cert from 'yourdomain.crt' and the private key from 'yourdomain.key'.
It must be like this:

BEGIN CERTIFICATE
lines of text between the Begin and End
END CERTIFICATE
BEGIN RSA PRIVATE KEY
lines of text between the Begin and End
END RSA PRIVATE KEY

NOTE: Do not include the extra text which is inserted by openssl

Save the text file as 'cert_with_key.pem'

Using the Java Keytool command:
keytool -import -file cert_with_key.pem -alias sitecertkey

You will also need to import the other certificates that came with your site certificate for the certificates to chain to a trusted root.
These will be attached to the email you received, or can be downloaded from:
https://support.sectigo.com/Com_KnowledgeDetailPage?Id=kA01N000000rfBO

Then follow the import instructions for the intermediate/roots at: https://support.sectigo.com/Com_KnowledgeDetailPage?Id=kA01N000000zFSe