If I understood you correctly, you don't want to change the default behaviour of CAPS LOCK, but only the behaviour of the number row when CAPS LOCK's state is on. In this case you can have among others the following options.
a) Change type by block
ALPHABETIC (or #_LEVEL_ALPHABETIC if you wish to have more levels) for those keys which should ...
Edit /etc/default/keyboard and change
This works for all users, and also when not logged in.
(Xmodmap settings get lost when you plug in/out external keyboards (including usb headphones with volume keys since they register as (very small) keyboards), unless the file is named ~/.Xmodmap, but ...