There are two security systems involved here, the remote central locking/alarm (which is activated from the removable fob part of the non-flip key) and the immobiliser transponder, a small chip or glass pill, glued into the key part of the non-flip key) (I don't know if the flip keys come apart in the same way).
Synchronising the remote central locking is relatively straightforward using the processes published here and elsewhere.
Coding the immobiliser to accept a new transponder is more difficult. Can sometimes be done with VAG-com, I believe, if you have the SKC (Security Key Code, I think), a separate code number on a tag that should have been given to you with the car. The SKC was discontinued in later cars, and recoding can only be carried out for these at a dealer with their diagnostic equipment.
If all you want is the remote functionality, you could buy an standard key fob and code it to the alarm. Still use your existing key in the ignition and the separate remote for the alarm/locking.
If you only have one key, then the risk involved in swapping the immobiliser transponder is higher. I'd seriously consider getting a complete key from the dealer, or at least getting them to add the new transponder to the immobiliser.
Oh, and the ebay places that will recut a key? They have a record of your key and your home address. I don't like the risks involved in that either.