I wish my customers to be able to use either postgres or sqlserver.
Thanks to hibernate they just have to edit the config file and restart.
The problem is the mapping of the unique identifier.

Postgres:
<id>
<generator class="native">
<sequence content = "myguid" />
</generator>
</id>

Sqlsever
<id>
<generator class="native" />
</id>

How should I modify my id generator to support both sqlserver and postgres?

Firstly, the first mapping option will still work under MSSQL. You should try it.
Secondly, I would consider using the new sequence generator(s) that were recently released as it will allow a more consistent id generation policy between the two DBs. It will result in more tables in the MSSQL database though (which, in my view, is a non-issue).