The new version of graphman supports pem based key's but it is unclear how to use them instead of p12.
Resolution
When you update the graphman query to get a key by alias like in the following example ,you can get the key in a pem format from a existing key.
querykeyByAlias($alias: String!) {
keyByAlias(alias : $alias) {
goid
keystoreId
alias
checksum
keyType
subjectDn
p12
pem
certChain
}
The following graphman payload should work when you replace the example value in the subjectDn , pem and certChain field for the data from the previous query example.