maybe there’s a way using lua scripting.
Write a script that creates an event that reads the current exposure parameter and copy the value to the system clipboard and another event that reads the value form the clipboard and sets the exposure parameter.
Then you can assign a shortcut to these events
a good starting point: Working with modules from Lua scripts in darktable - #9 by wpferguson
darktable lua documentation - darktable.register_event
export to clipboard · Issue #11558 · darktable-org/darktable · GitHub