It won't be as simple as using BASS_FX, but it is possible to use other time/pitch-changing libraries with BASS. You would still create a "decoding channel" source like you do with BASS_FX, but you would then create a custom stream (with the same sample format) with BASS_StreamCreate instead of using BASS_FX_TempoCreate. The custom stream's STREAMPROC callback function would fetch data from the source (via BASS_ChannelGetData) and process it with the library before returning it to BASS.