Yes you can, but it depends on the 5 speed freewheel, the max cog you are running (more below), and how much you are willing to spend to get there.
Once upon a time Shimano made the 74XX SIS rear derailleur with an actuation rate of 1.9:1 instead of the SIS standard 1:7:1. This meant that the cable pull for DA 74XX downtube and eventual STI shifters pulled less cable to work with 6, 7, and 8 speed Shimano freewheels and cassettes. It also meant that non-DA downtube and STI shifters that pulled more cable opened up some interesting index shifting possibilities.
It starts with Sheldon Brown's freewheel and cassette spacing cribsheet.
https://www.sheldonbrown.com/cribsheet-spacing.html
There used to be a blog by Art's Cyclery "The Science Behind the Magic" that is no longer up that gave the cable pull for SIS shifters.
The old alternative was Cycling UK's Guide to Rear Shifting
https://www.cyclinguk.org/cyclists-l...gears/shimergo
If you are running a 28T or less max sprocket on a 5 speed freewheel it does open up the possibility of a 7 speed SIS shifter (7 speed STI/brifters, Bar end, Bar end on a Grenevelle, 7 speed SIS thumbshifter, or even a 7 speed SIS downtube shifter on Kelly Takeoffs that are hard to find) as the math Shimano SIS 7 speed cable pull 2.9mm X 1.9 actuation ratio gets you to a 5.51 5 speed spacing. The worst case 2 decimal calculation of 2.94 X 1.9 equals 5.58. The .003" per shift over 4 cogs is probably negligible and within the Centeron pulley slop. Again the caveat is a 28t max pulley which is 2t over spec and probably achievable on an '82 Nishiki touring bike with horizontal dropouts.
If you go to a larger max cog, you might be able to use the much maligned Wolftooth Road Link. With a 5 speed and a 14t small and 32t large cog and a 50t/40t chaining or any other 10t chainring difference you are at a capacity of 28t which happens to be within a Dura Ace 74XX spec. In your setup I really doubt a noticeable drop in 'shift performance'.
John
Edit added: You will only use 5 of the 7 SIS shift positions on a 2x5 setup.