Is it possible to make update package with only userspace components?

If I make some modifications to device tree, kernel or other base files and would like to keep them while adding some user space components afterwards, is it possible to use firmware selector?

Can firmware selector be used to make such update package, which does not contain and overwrite kernel parts?

If this is not possible, would someone else but me be interested in it?

It's of cource always possible to add these components by simply logging in and installing them using System -> Software, but a solid package would help installing more than one device.

Nope, you have to do full build. Firmware selector (or owut or imagebuilder) recomposes kernel image and packages.