Here is how I do it. Say I have a file on the macos side I want to get
into netbsd. The file is called 'Xproggy.tar.gz'  It is on a drive which
is named HD500 and in a folder on that drive called Xprograms.  I want
to copy it into /home/cruller on my netbsd partition.  Here is what I do
from the minishell of the installer.

cpin "HD500:Xprograms:Xproggy.tar.gz" /home/cruller/Xproggy.tar.gz

Note that I put quotes around the macos path and : (colons) for path
subdirectory(read folder) markers, but on the netbsd side quotes are
_NOT_ required and the subdirectory marker is a / (slash).

I have a Zip drive that I DL files onto in the MacOS.  I named the drive
D  and a folder  X to save a few keystokes when typing this stuff into
the minishell.   I also copy the file name right from the finder & past
it into the minishell at the apropriate spot.  This serves me two ways:
1) I don't have to type it . 2) it eliminates errors because of criptic
file names e.g.: (Xlockmore-0.5.1-bin12.tar.gz) Is that a 0 or a O???;
so here is what I get:

cpin "D:X:Xproggy.tar.gz" /home/cruller/Xproggy.tar.gz

It would be great if I could find something that copies the whole path
in the MacOS so I could save even more keystokes. :)

I hope this helps, Good luck

Gerald