If you want to override a named command in main.cf, normally you can specify in the master.cf as override option (-o) to the command.
There is one quirk here though. It cannot contain any white spaces. The manual page explain this clearly. However, it is silent when it comes to parameter values that contains whitespace. There are no examples as well.
The simple trick here is to replace white space with comma as well.
For example:
….. -o smtpd_client_restrictions=check_client_access,hash:/etc/postfix/access
Please see the comma after check_client_access.
But when you use this parameter in main.cf, it will be as below
smtpd_client_restrictions=check_client_access hash:/etc/postfix/access