I looked in the Firmware a while ago, and the firmware does support arbitrary module lengths using the M1025 [X|Y|Z]<length_in_mm> commands.
You might try emailing support@snapmaker.com to see if they still have any extended Z axises in stock, or if they’re willing to sell you the parts. It’s been hit or miss if they still have Original parts available. The link above was a request for SnapMaker to open source the Original schematics. AFAICT, they never did.
I recall people doing some big mods to their linear modules to make them quieter and more powerful. The link does have a lot of links to other forums posts that would likely be of interest. There were also several posts on the forum about how to safely disassemble and reassemble the linear modules. If you’re careful, you should be able to disassemble, catalog parts, and reassemble. Once you’ve got the catalog and dimensions, that should be enough. AFAICT, all of the parts they used to build the SM are available on Alibaba. If I had to guess, I’d say the linear module body is the only thing you can’t buy there.
[Edit to add] My original plan to get a larger work area was to use the snapmaker to print a RepRap.