We have that issue also, what I have done is the following:
a) Use a custom field stating the Third Party warranty vendor, so it appears on the Summary page.
b) Modify the warranty end date (on the edit asset page) to reflect the new one. And make sure the field is locked so it does not get scanned again...
As I use Lansweeper as the main tool for warranty management, this assures that they get on the correct reports, etc.