Sorry sir, you're right...
But I still expect function could do this without converting it to char datatype. It's important -as I said- when you want to add an interval value and Add/Subtract another time value and make some comparisons. Definitely you don't like something like this:
*****************************************************************
L_Delta_All:=To_Timestamp(to_char(F_TIME_IN_OUT+F_MARGIN_IN_OUT_AFTER,'hh24:mi:ss'),'hh24:mi:ss')-
To_Timestamp(to_char(F_TXN_IN_OUT,'hh24:mi:ss'),'hh24:mi:ss');
*****************************************************************

The previous statement is simple Add Time portion to Interval and subtracts the time portion from another field.
********************************************************************
L_Delta_All:=(F_TIME_IN_OUT + F_MARGIN_IN_OUT_AFTER) - F_TXN_IN_OUT;
********************************************************************

So there's anyway in order to make the previous statement in simplest way.

Thanks.