i got one account with whm, 1000mb size.then when i come to whm, i click "list account" my current domain is shown there with the 1000mb. Now if i want to add one domain with 200mb, do i need to modify my current domain account to 800mb, then add the 200mb?? i hope you understand, i really no sure how to describ it

Hello,

If your reseller account allow "overselling", there is no need to modify your main account. If not, I suggest you to allocate a package with small quota to your main account, otherwise you cannot resell much space to your customers.