Are you talking about an obturator? If you are, they are relined just like any conventional prosthesis. The doc needs to be careful about generating the impression to keep material from going in places where it doesn't belong.
Often, because of the altered muscle tissues and scar bands, the final layer of the reline process is done chair-side with a softer, self-curing material.
An easy way to box and pour the impression is with alginate; see the post under general discussions "box and pour quick and easy" this will allow you to block out undercuts in the appliance if needed. A reline is the same- remove and replace acrylic as needed, just be carefull of major undercuts.
You won't know till you see the appliance and the impression of the cleft. It may be a hollow bulb or it may be a soft-lined chunk of silicone. It may be a simple hard reline. Conventional processing for each of the above techniques can be elaborated on if you need.
I did an article a few years back on an the all soft 'hallowed' bulb obturator. I can email it to you if you'd like. Or, you can go to dentaltown and do a search for it there.