How can I insert a receptacle into this junction box?
Box Fill
Box fill might be an issue.  There are 8 current carrying conductors, if they are 14 AWG, that's 16 cubic inches (18 if 12 AWG).  Plus another 4 cubic inches for the receptacle (4.5 for #12).  And another 2 cubic inches, if there are grounds (2.25 for #12).
Equipment Ground
If there aren't any grounding conductors, you'll either have to install a 2 prong receptacle or a GFCI receptacle with a &quot;No Equipment Ground&quot; label.
What's What
You'll also have to figure out where all the wires are coming from, and where they are going.  You can't just tap circuits all Willy Nilly,  you have know what else is on them.
Which Neutral
As for which neutral to use, you should defiantly use the grounded (neutral) conductor from the same circuit as the ungrounded (hot) conductor you're using.  You could end up overloading a neutral, or other problems if the circuits are out of phase.