This will largely depend on the type of ISA that you currently have. Most variable rate ISAs will offer the ability to make further contributions each year. While there are exceptions to this rule, it is more likely that you will be able to add to the account.
Fixed rate isa accounts are another story completely. The situation is actually the opposite of a variable rate ISA. There are exceptions that will allow a person to top up their account, but in most cases, further contributions are not available.
Most experts would recommend transferring the ISA out to another provider to take advantage of new interest rates and then adding the yearly contribution to the new account.
One of the exceptions to the fixed rate ISA rule mentioned above is the Northern Rock ISA. This ISA offers a 4.5 percent interest rate for five years, and offers a thirty-day investment window to increase the contribution.
Yorkshire also has a fixed rate ISA that offers a similar window where investors can add their yearly allowance to the account.