SQL & PL/SQL

Is it possible to create custom "Group By" functions?
I want to create something similar to MySQL's Group_ConCat.
I have creted a function that works on single row queries by feeding it a SQL statement (with a where clause like
Select dogs,
RollUp('Select Foo from Bar where FBar = ' || mustard)
From hotdogs;

But it would also be nice to see
Select dogs, Rollup(mustard)
From hotdogs
group by dogs;

(Rollup would create a comma separated list of the paramater given.)

Thanks.
Andy