After a snapshot there is often this problem, that you have to add addresses to see your full balances again. If I exactly know that there should be for example 20MIOTA in that seed, would it be possible to send this amount to a new address even if it is not shown on my balance?
The money is in your addresses, not in the seed. So if your wallet does not see the funds, it cannot send them.
However, it is possible (with IOTA API) to scan addresses without attaching them until you reach a certain amount and then send them to another address. The current GUI wallet does not support this, though (but perhaps the new UCL wallet will? At least think they will build in some features to handle snapshots more seamlessly)