2012 works with New-NetRoute. As far as I know , 2008 cant use it (only wmi) .
so no easy way here . do you have script that can transfer PersistentRoutes from 2008 server to 2012 ?
Sorry, no.
You could use netsh to dump the routes from the 2008 host.
netsh interface ipV4 show route store=persistent
The output is formatted such that getting it into CSV should not be a problem. You could then use New-NetRoute with the CSV file.