I think it is already there; try using something like:

$g_smtp_host = 'host:port';

and let me know if that works for you.

if its not, it should not be a problem since phpmailer supports it...

I didn't try (PHPMailer doc confirms the format you give me). Maybe this format (host1:port1;host2:port2) could be explained in the comments of the config_default_inc.php file?

Ok. I am going to add this sentence:

You can specify a different port by using this format: [hostname:port]

but please test the feature and let me know if it works as expected.

I didn't forget ;)

I need time because this feature is not for me (I need to configure my SMTP server to listen on another port)...

Once we confirm that the port and multiple SMTP server support works, we should document it in config_defaults_inc.php or add a link to the phpmailer website where such format is described.

It's okay.
'host:645' => timeout (no SMTP server listening on the port 645).
'host:25' => okay and mail received.

I refined the documentation for the option in config_default_inc.php.

Feel free to open a new bug if this does not work.