You can use the automatic transmission in the base RSX to select what gear you want. There's a setting on the transmission that lets you choose. It also functions completely normally when you tell it to.
Though... if you're thinking about modifying your RSX, it's recommended not to get an automatic transmission. You still can modify an automatic, it just has certain things you have to work around... won't get into it deeply. I'd suggest learning to drive a manual, it is not hard at all.