Using external DAC with preamp with built-in DAC

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.

Of course it will work.  But try it both ways to see what you like best.

I hooked up my old Arcam DAC to the line in's and did an A/B against the resident DAC in my integrated. The resident DAC in the integrated uses a lower end ESS chip compared to the TI chip in the Arcam, but the Arcam DAC is close to 15 years old. The lower end but newer ESS chip sounded better to me.. 

I just got a Quad 33 all analog preamp, using an outboard SMSL DO200 Pro DAC with it. The signal path is all balanced XLR. The Quad has MM/ MC phono, a good headphone amp, and useful tone controls, for well under $2K. I had a C-J PF-R before this, and I like this better.