I think you have 2 options:
You could look in the web.config file, and copy the
<rewrite> section, then paste into the web.config file on the new system.
Or, you could use appcmd to export the rules to a file, and to import them on the new system:
appcmd list config "websitename/appname" -section:system.webServer/rewrite/rules -xml > rewriterules.xml
appcmd set config -in < rewriterules.xml
You can also export any global rewrite rules using:
appcmd list config -section:system.webServer/rewrite/globalRules -xml > globalrewriterules.xml
The import command would be the same.