Hello
Client import, is an implicit client deletion, no way to make it selective or incremental outside of what is defined in profiles.
But there might be a way to customize this... I'm using Oracle scripts to export / import users & roles when doing my refresh, because this action easily fits in a more global script, and this kind of import does not delete existing data.
Roles might not be an issue as far as you can save & import them using CTS orders.
Before the copy delete through SU10 all the users you do not want to keep, export all users that are left at DB level.
After the client copy process import back at DB level what's has been exported. With DB import no deletion takes place, thus you will only add (or overwrite depending on selected option) the exported users.
This import / export could also be done using R3trans script.
I can provide you with the Oracle datapump command if you're interested, I've just created it based on the list of tables that are copied by client copy when using SAP_USER profile.
Regards