I should note that you're asking two slightly different questions: how to determine whether a hex is within the sector or not is a different problem from generating all the hexes within the sector; the generation problem is more challenging, and it may depend on what order you want to generate them in. (Also, note that 'all' here means infinitely many, so you'd probably want a stopping procedure too!)
As far as the testing problem - 'is this hex within the sector?' - there's a relatively straightforward procedure, though not quite the simple equation you might be looking for. Since hexagons are convex, then the hex is entirely within your sector if and only if all six of its vertices are within the sector; and because a sector is the intersection of two half-planes, then testing a point against the sector comes down to the AND of two half-plane tests for the two defining half-planes. For instance, if you have an upwardly-oriented 90-degree sector, then the two half-planes in question are the two half-planes '$y\gt x$' and '$y\gt -x$'; you'd simply confirm that each vertex of your hex satisfies both of those constraints.
To go much further than this, the problem needs to be still more precisely stated; for instance, a sector isn't uniquely specified by just its angle; you need both a center or vertex point for the sector (which it sounds like will always be the origin in your case, but that's not clearly specified) and two angles - either the angles of the left and right sides, or a central 'facing' angle and a spread to either side of it. There are likely to be slightly smarter tests available, since depending on the specific sector angle only a couple of vertices will need to be tested for most hexagons (rather than all six), but those tests depend a fair bit on the problem's precise specification.