And just in case you're still questioning the paint/compound thing. You can't use paint as texture additive to plaster, it just won't work. You always texture first, then paint afterwards. There are some caveats to this, but I won't confuse you with superfluous info.
What I would do is to mix sand of a similar grit to the existing texture into your drywall compound. Add a small amount of sand, mix well, and try it. Adjust the consistency of the drywall compound with clean water, add more sand if necessary. When you're happy with the grit and consistency, load the side of the float with an even bead of your texture compound and wipe it onto the surface.
It will take you some practice to get the size of the bead of texture on the float right (probably about 3/4" ). Set the float on it's edge at a severe angle, and as you move across the area, you flatten the float to the wall. This pulls the bead of texture off the side of the trowel in a metered amount and produces a uniform thickness.
Here is a tip, stay on the smooth patched surface until you've created an acceptable texture pattern. Practice, practice, practice! If you don't like your results, scr-ape the texture off with a drywall knife and start over. That is the great thing with drywall, if you don't like what you're doing, wipe it off and start again. You will probably want to practice on a scrap, just so you don't damage the wall with too many removals of the texture. Once you're satisfied with your results, then you can feather in the new texture to the old. You will need to texture out wider than your patch, to help blend in the area.
When it comes time to paint, put at least one good coat of primer over the area before you apply your top coat, this will help to lessen the shadow you'll get between the previously painted surface and the new work.
Good luck, let us know how it goes.