Add the resources to the file /etc/dt/config/language/sys.resources. (You may have to create the file.)
For example, if in /etc/dt/config/C/sys.resources you specify:
AnApplication*resource: value
then the resource AnApplication*resource will be set in each user's RESOURCE_MANAGER property at the next login.
For more information about Style Manager resources, see the dtstyle man page. For more information about Mailer resources, see the dtmail man page.