I assume its the plastic expansion tank you are talking about if so.. you could try some kind of sealant... I wouldnt really recommend it. Im sure you could pick up a good expansion tank from a scrap yard which would be a lot more of a permanent solution.
I just wouldnt have any confidence in something like that being patched up... you'd never know when it could fail. And things like that usually decide to give way at the
worst possible moments.
I have been involved in welding polythene and other plastics for years, at best you will only smooth over the surface with heat. The crack will still be on the inside and will work its way through to the outside again. This will be caused by the pressure cycling as the water heats and cools on each journey. Repair may last a day or a year but it will fail.