adding NS servers to named

Well,  

 I think you want to edit the definition files in /etc/namedb/............................

An add you data, change the serial No,  then reload named..

But you really don't give much to go on, like what OS you on and exactly what it is you are trying to do ?

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like for example I have "domain.com.db"
but I want to create a new one with different NS
so shall I create something like "games.domain.com.db"
and add custom different NS ?

First, you must register the new NS with your Domain registration service.
whois domain.com.  See who the registrar is. and then go through their interface to register the NS.domain.com with IP x.y.z.a
Then make sure the named instance on this server is functional and has the current configuration (presumably you have a master slave setup) Or are you using named with mysql backend?

Given you mentioned a flat file, Once you've confirmed the data is present, updates show up.
AS was pointed out, edit /var/named/domain.com.db update serial (increment by one, or change the data to yyyymmddii where ii is an index allowing for 100 changes per day.  Using the date format provide for simpler diagnostic.  

host -C domain.com will return the date/index when the last change was made.

add
    IN NS newns.domain.com.

all the NS servers will be notified of the change. If you have a master/slave, the change will trigger an update.

arnold, all db files need to have a different serial number right?

the master *may* signal an update if notify is turned on.  the slave only receives the update.

the serial number *may* be the same in all files.  but when a change is made to the file, the serial number should be "serial number + 1".  this change is only made to the file that needs to be updated.