The Classic FTB Wiki does not cover Knapsack, since it does not exist in Classic Feed The Beast (Minecraft versions 1.4.7 and earlier).
You may find information about Knapsack on our sister site,

The Knapsack is a useful tool but it has problems yet to be addressed (9/18/13). the Knapsack adds a smaller inventory to your armor tabs that you must interact with yourself, items will not be placed in the knapsack upon picking them up even if the item already exists in the knapsack. The main problem with the knapsack though, and probably the main reason it hasn't seen much use, is because it despawns when you close the game and it takes any items it contained with it. To fix the despawn, click on the "Armor" tab. The knapsack tab should then show up.

Crafting the Knapsack is simple enough but costly depending on your location; it requires six pieces of leather, two tough iron tool rods, and a gold ingot.

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