PL/SQL I am using a INSERT as SELECT from various tables

PL/SQL I am using a INSERT as SELECT from various tables

In PL/SQL I am using a INSERT as SELECT from various tables that uses many
functions and relations. the select returns about 10 rows at a time.
The query looks like
INSERT INTO EIM_TABLE(certain columns)
select a,b,...,fn(x,y,)
Now i need to collect the value of what was returned by fn(x,y) ,
concatenate each of the values into a single variable and carry out my
further processing based on this.
Please suggest the best way to this. Performance is a highly critical
criteria so want to avoid unnecassary overheads