When customer requires list of technical staff that will be involved in the project;
What is the proper method to give the info of the sub-contractors/freelance workers that will be involved in the some stages of the project ?
As a rule of thumb: transparency and openness can go really far in terms of building trust between you and a customer. And trust is definitely kind of relation you want to build with your clients.
Considering it is an important piece of information for the customer or they directly asked about it I would just openly state that some people in a project team are part of staff and others work as contractors/freelancers. It isn't that uncommon in our industry to be afraid to say this, and transparency definitely helps to build trust.
The other approach would be to treat this information as irrelevant and just not share it at all unless directly asked, but for some reason you are asking about it, thus you believe it is not the case.
Note: I do consider that there aren't and formal (or informal) agreements in place between you and the client which state that you aren't allowed to have contractors/freelancers working on this very project.