1.If you have a relatively simplistic mapping, you can embed a oracle/db sequence.nextval call hidden in the source qualifier.
2.you can embed db/sequence call in a sql tranformation too. But know that it would be anti-performant.
3.you will be able to achieve a sequence generator behaviour using a persistent variable too, but there are limitations and downsides.
Depending upon what you are trying to do and where you are getting stuck, you might want to repost/edit our question.. and perhaps get a more direct answer.