How can I add *.mydomain.com to httpd.conf so that I will be able to add trhough CPanel subdomains? Now I'm doing it manually, because CPanel adds subdomains to the end and I'm moving those code added by CPanel up, before *.mydomain.com code. Is there a way to force CPanel to add new subdomain code before *.mydomain.com statement?

Do you have access to your DNS zones? I am guessing not. Ask your host to help you create the wild card dns.

How dns zones are related with httpd.conf problem I've described? ;)

I don't know if there's any way of doing that for subdomains, I think the code that creates the subdomains may be in with the cpanel code somewhere so you can't read/edit it. For regular domains you can take a look at the /scripts/wwwacct file for the VirtualHost templates.

Well to do wildcard DNS you need to amend both your DNS zones and your Apache config.

But I have to admit I misread your post.

Are you talking about adding the lines during account creation? If you are, you would need to look at editing /scripts/wwwacct and chattr +i it after you are done or your chances will be gone next update.