The appeal takes a long time to set up, so you get plenty of time to get your evidence in, but the DWP can get on with reviewing its decision straight away without waiting for all that evidence.
There's nothing to stop you giving them more evidence and asking them to take another look, but in the meantime, their having decided on the basis of the evidence they have so far not to overturn their decision doesn't mean that the evidence you submitted will not be considered at tribunal.
If you sent additional evidence to the DWP it should be in the appeal bundle. If it is not, or if no evidence has been sent to the DWP, make sure that CAB send the evidence to the tribunal, so that it will be included in the bundle before the hearing.
