Apple does not sell storage upgrades for its Mac Studio mini-PCs. However, it is now possible to purchase 2 TB and 4 TB drives for M1 and M2 machines for up to 8 TB of total storage thanks to a third-party product called Studio Drive.https://www.notebookcheck.net/Apple-Mac-Studio-storage-upgrades-now-available-through-Studio-Drive-Kickstarter-campaign.902144.0.html
If this indeed works (it should, but a scam is always possible, especially on crowdfunding sites), there is still the possibility of Apple disabling them via a firmware update - which would scrap your upgrade.
I wouldn't trust this personally, Apple controls their hardware and software to the end: this is why it both works so well AND costs us so much at purchase time. An external TB 3/4 dock with drives is probably a safer and time-proof option.