There are helical focuser add-ons available for ezxactly this. The direct version is hard to find now (it's M50 thread or something obscure like that), but there are other options on eBay/AliExpress etc. They look like this:
https://www.ebay.com.au/itm/SVBNOY-1...0AAOSwTmRdPAsd - that one has an M42 fitting, so you'd need to have a custom adaptor made from M42 to the odd SW thread.
I have one of the original helical focusers that were around for a few years and it screws right onto the back of the SW 9x50 finder perfectly with no adaptor - haven't seen them in a while. I'll post a picture later.