A WAV file contains a header plus a lot of audio samples; the header holds some parameters that apply to all the samples, it includes number of channels, number of bits per sample, and sample rate.
You can concatenate audio samples from different files as long as the header information matches (except for the length of course).
When the headers don't match, some conversion is required. I have never done such conversions, however I recommend you read, understand, and possibly use (some of) the code in this article:
C# WAV file class, audio mixing, and some light audio manipulation[
^]
And you will have to watch out for volume problems, one file being much louder than another...
:)