I don't know what you exactly mean, but sbnk can only be converted with the samples. Which are in the swar files. In mkds course modifier you have to open the sdat. Open the sseq then and it will be played with sbnk. It also supports looping.
If you grab a HGSS rom and try playing the GB Sounds themes in VGMtrans, you won't hear anything because the 8-bit sounds it's using are direct, not sampled. And when you log to DLS, it's the same result.
They've worked in NDS Music Player since @KazoWAR added direct sound support.