Question:
May one deliberately eat much more than usual before a fast in order to avoid feeling hunger during the fast?
Answer:
In general, no.
The Poskim rule that one should not intentionally overeat before a fast in order to avoid experiencing the affliction of the fast. Doing so undermines the very purpose of the fast, which is to feel a certain degree of deprivation and thereby be inspired to repentance and introspection.
Nevertheless, this limitation applies to a healthy person who is merely seeking to make the fast easier. If a person is weak by nature, or knows from experience that he will have difficulty functioning unless he eats more than normal beforehand, then he may do so. In such a case, the extra food is being eaten for health and strength rather than for the purpose of circumventing the fast.
Conclusion:
One should not overeat before a fast merely to avoid feeling hungry. However, a person who needs additional food for health, strength, or proper functioning may eat more than usual before the fast begins.
Sources:
Kitzur Shlah; Nahar Shalom 550:2; Elya Rabbah 563:1; Bigdei Yesha 564; Kaf Hachaim 549:11; Piskei Teshuvos 564:1.