If you sistered the current joist a couple times you would in fact have a beam....
Sistering a joist is when you stick another joist beside the current one and glue and screw it into place. When one builds a beam, say out of 2X10's typically a layer of plywood is placed in between the two 2x10's, all glued and screwed together.
You could sister the joist in the manner one would build a beam....meaning current joist, glue and nail piece of plywood to it, stick another joist up there glue and nail that to the plywood, you have in effect built a beam.
Now whether or not that beam will be strong enough is a question I can't answer, since I don't know what (wieght) your post upstairs is holding, but that post has a limit, and if the beam i.e. sistered joist and new wall can support the amount of weight that your post is rated for you should be good.