Custom actions for mock chroot init

[Michael Ivanov]

Greetings!

I hope this is the right place to ask questions about 'mock' buildsystem.
Is there a possibility to add custom actions to mock chroot tree initialization?
I need a specific user and group to exist in rpm build environment. Currently I
do it manually using --shell command, but as soon as I run src package build
forgetting --no-clean option, everything is wiped away and recreated automatically.

And another one. Is it possible to request some files to be copied into chroot
tree after initialization? I found 'files' config option, but it looks to me that
file contents is to be embedded in cfg file: and this is possible only for short
text files.

I expected to find an option to specify post-init script to perform such actions,
but did not succeed

Best regards,
-- 
Michael Ivanov