Yes dzug is quite right. You cannot "add" to an existing ISA once the tax year has ended and any new money you invest will constitute a new ISA.
Fixing your ISA rate can be advantageous if you do not need access to the cash for the period you tie it up. At least you know what you will get. But be careful - some of the rates offered are truly pathetic even for a fixed term investment.
Also bear in mind that (provided it is not a fixed term ISA) you can move some or all of your existing ISA investment to another provider (provided they accept transfers). So it may be worthwhile considering this next April when you come to think about your investment for 2013/14.