I want to make a couple minor changes to a HL1 era DoD (1.3) map. I understand that "Hammer" is used to do this?

Also, is there a way to take the existing bsp file and import it to edit it?

Thanks!

Yes....ish.
There are programs that will decompile a .bsp to a file that hammer can use to edit a map.
However this places you in dodgy legal grounds regarding copyright. What you cannot do is edit the map then release it without the original author's permission as this breaches artistic copyright (and is also a pretty crap thing to do anyway - professional courtesy is important).

I see... Thanks for the info. I just want to make a change to fix an error, but I don't know if the original creator will respond to an email.

Thanks!