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