Using external DAC with preamp with built-in DAC

Most high quality preamps do not have an internal DAC. I Don't recommend them, even if there is a slot. In general, you will get better sound by isolating functions. So, a preamp only being a preamp. Depending on your budget I'd looks a used Conrad Johnson, Audio Research or VAC preamps. These and other high end companies may outstanding preamps that will last for decades. 

Without knowing what make/models you have been looking at, I would caution you to not confuse an external DAC and/or streamer that also can operate as a preamp (i.e. has a volume control) with a true preamp.  Typically their focus is on the digital side and do not sound as nice.  An example of this would be a Cambridge Audio CXNxxx series as aposed to a Hegel H120 or PS Audio Stellar Gain Cell, which have an analog preamp section and the DAC is an "addon". 

Absolutely yes.

Yes, you can use it through an analog input as others have said. But absolutely plus one on a pure preamp will sound better!   I don’t know your price range, but an Audionet Pre G2 was my first ear opening foray into amazing sound.
 

@OP - I suspect you are looking at preamps at the more affordable end of the market as it's those that tend to have built in DACs.

The majority of high end preamps are analogue only.