If the sources all have the same sample format (eg. stereo 44100hz or whatever), then you can actually do this without BASSmix; you just need to use BASS_Encode_SetChannel at the end of each source to move the encoder to the next source.
If the sources have varying samples formats, then you should use BASSmix (and feed the mixer to BASSenc) so that they are resampled to a consistent format. Here's an example of using BASSmix for gapless playback (or encoding)...
www.un4seen.com/forum/?topic=6159.msg41390#msg41390