Expanded Knowledge( Expanded Psionics Handbook, p. 46)
You learn another power.
Manifester level 3rd,
Add to your powers known one additional power of any level up to one level lower than the highest-level power you can manifest. For example, a 7th-level psion gains either a new 1st, 2nd, or 3rd-level power. You can choose any power, including powers from another discipline's list or even from another class's list.
You can gain this feat multiple times. Each time, you learn one new power at any level up to one less than the highest-level power you can manifest.