You should be able to parse out what you need using the tags in the .bam. 10xGenomics' website says what tags they add.

https://support.10xgenomics.com/single-cell-gene-expression/software/pipelines/latest/output/bam

Going backwards would also involve parsing the tags, because you have to make two fastq files, and a simple bam -> fastq pipeline won't do that correctly.