See this
link on pcunix.com SCO FAQs.
Beginning with 5.0.6, you can add the default route in /etc/default/tcp. Just modify the GATEWAY= line, and /etc/tcp will read that. DON'T DO THIS ON PRIOR RELEASES; /etc/tcp doesn't look for that until 5.0.6
Prior to 5.0.6, you have choices. You could add the commands to a start-up file (/etc/rc2.d/S99route, for example: it doesn't exist, but you can create it) or modify the /etc/gateways file (see 'man routed'for the syntax of that file). The disadvantage of /etc/gateways is that it is used by routed, so if you are not running routed, that won't work.