Adding data to Array field of Storable class

Which opcode can be used to add data into arrays fields of custom storable class ?

Suppose i have a class /purchased_product which have one array element as PIN_FLD_CYCLE_FEES.

Earlier i was just entering data into my class fields but now i want to add data to my array fields too of that objects of storable class.

I tried to update it but i was not able to do so using:

r << e 1

0 PIN_FLD_POID           POID [0] 0.0.0.1 /purchased_product 1742825 0

0 PIN_FLD_CYCLE_FEES    ARRAY [1] allocated 20, used 6

1     PIN_FLD_CHARGED_FROM_T TSTAMP [0] (1391756400) Thu Feb  6 23:00:00 2014

1     PIN_FLD_CHARGED_TO_T TSTAMP [0] (1423206000) Thu Feb  5 23:00:00 2015