IMHO, the simplest way would be to change your DUB-JFK rtn into a DUB-JFK // SFO-JFK open jaw, and add whatever one-way NYC-SFO fits your budget/schedule needs best.
It's difficult to be precise without knowing the rules of your existing fare but it's likely to be changeable. For example
- a change fee of EUR180
- any difference between what you paid and the fare in force today
e.g. if you bought this today, DUB-JFK rtn is EUR523 and DUB-JFK // SFO-DUB is EUR562
You might even get work to stump up for WT+ on the return if that's in your travel policy.
Make sense ?