For database first, it maps timestamp already to DateTime for postgresql, so you're looking for the modelfirst variant I recon?
You can do that by altering the driver sourcecode, and as v4.2 won't receive new updates that's not a problematic issue. In Postgresqldbdriver.cs line 231, you'll see the rules for datetime, where Date is the first choice. Switch the line with the one with Timestamp, compile it (change the strong key in assembly info) and the DateTime type should now result in Timestamp fields in the DB.
A type converter won't work here as Timestamp already resolves to DateTime.
In v5.x you can specify the default type so that would solve it without altering the driver.