My problem is quite a simple but I am not sure what is the best solution. I wish to send an email from a MODX snippet with different account/pass that is stored in MODX settings. In other words, mail_smtp_user and mail_smtp_pass should be overrided during the execution of the snippet.
I will then use $modx->mail to send an email normally.
I found a better solution.
I made new pass/user settings to the secondary email account and added the following code to the snippets that are using this secondary account.
Works well now.