Rather than converting from pfx to pem format, why not just use a pfx?
With the way that pfx files are formatted, copying and pasting from a terminal is not possible, however if you can get the certificate transferred over FTP, it becomes much simpler :-)
The certificate has to have the full chain in it, and a passphrase.
conf t ip ftp username <ftp username> ip ftp password <ftp password> end copy ftp flash:
enter server name
enter source file name
enter destination file name
conf t crypto pki trustpoint <trustpoint name> fqdn <f.q.d.n> subject-name cn=<f.q.d.n> revocation-check crl rsakeypair <trustpoint name> exit crypto pki import <certificate.pfx> pkcs12 flash:<certificate> <passphrase> exit wri mem
to show the certificate
show crypto pki trustpoints status