You should be able to do the following from cli:
crypto ca export <TrustPointName> pkcs12 <password>
Save the output to a text file then convert it using openssl (available for Linux or within Windows if you install the optional Windows Subsystem for Linux (WSL)).
openssl pkcs12 -in TrustPointName.txt -out key.pem -nocerts -nokeys -passin pass:<password>