Maybe this is not neccessary. The SSH check uses this plugin: Monitoring Plugins - check_ssh. The manpage says the tool accepts a parameter
--extra-opts=[section][@file]
Read options from an ini file. See
https://www.monitoring-plugins.org/doc/extra-opts.html
for usage and examples.
and Monitoring Plugins - Extra-Opts says
The initial use case for this functionality is for hiding passwords, so you do not have to define sensitive credentials in the Nagios configuration and these options won’t appear in the command line.
Unfortunately they don’t supply a nice example of how the ini file must look like and whether it really accepts a password as parameter.