Just edit the custom template you created ( /etc/e-smith/templates-custom/etc/postfix/main.cf/90myconfig , for instance) and expand the template for the target file ( expand-template /etc/postfix/main.cf ).
Afterwards, the changes you made shall be present on /etc/postfix/main.cf. You can check this file to make sure the changes are reflected correctly (no syntax / formatting error…)
Remember the name of the template file matters at least to set the order in which the templates are processed to build the final target file.
Once you have it fully working do you mind to post a step by step howto here, so anyone else facing the same issue can use it?