external offsets

Can you elaborate? I don't see how you can be using matrix_kb with a handwheel. matrix_kb works with keyboards (and other matrix-wired switches)

Do you want to type in an actual number, then press "enter" and have that applied as an offset?

Or set fixed offsets depending on which of the keys 0 to 9 has been pressed?

One way to do this would be with a mux component. If you feed the
output back to the input then you get a sample-and-hold type
component.

If you connect each of the 10 inputs to a "sel" pin of such a mux,
then each would select from input 1,2,4,8,16.....
However this might be a bit unwieldy as a 10-input mux has 1024 input
values (of which you would only use 10). This isn't a major problem,
it just means that there are an awful lot of unused HAL pins.
mux_generic.9.html

I have looked through the available HAL components and I don't see one
which does what you need, ie output 1 if pit 1 is set, 2 for bit 2, 3
for bit 3. I might add one, but in the interim here is a custom
component to install with
halcompile
Save this code as selector.comp and install as directed in the link.
(short version: sudo halcompile --install selector.comp)

Wire the keyboard output pins to the select-nn pins, set the value-nn pins to the required offsets and wire the output pin to the external offset.