dyanamically add field form an internal table

Hiii

there are data that i am extraction from database from FAGFLEXT tabel more exactly

in that table i have field KSL01, KSL02, KSL03, KSL04, KSL05 .... until 16

on my selection screen i have a parameter P_period

if ever i specify 05 on my selection screen i have to add KSL01 + KSL02 + KSL03+ KSL04 + kSL05

if ever i specify 02 on my selection screen i have to add KSL01 + KSL02

Note that i will have an internal table of fype FAGFLEXT and also i am treating multiple record.

The first thing that i am thinking of i building a routine or macro so as to put

if field P_PERIOD CS 01 then value = KSL01

if field P_PERIOD CS 04 then value = KSL01 + 02 +03 +04

But there will be too much if and this is not performance wise