BAM to gene expression matrix (UMI counts per gene per cell),10X
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.