Is a dhea supplement safe to use for an adrenal imbalance?

Safe, but ineffectiv. If you truly have adrenal insufficiency, dhea alone is not adequate replacement, and you will need additional hormones including glucocorticoids and mineralocorticoids. "adrenal imbalance", "adrenal fatigue" etc are really pseudo-diseases, not well characterized, and it probably wont hurt to take dhea.