I have an older home with a mix between romex and pbx wiring. I was
testing the outlets and found that there are 2 that are not grounded,
which are fed with pbx. I found the junction box in the light fixture
that they are connected to was replaced w/ a blue plastic box and there
is nothing that grounds the 4 pbx wires going into it together (source
feed, switch & 2 outlets)

My question is, should I attempt to solder a common ground wire between
each pbx wire, or replace the junction box? I'd rather not replace the
box, but I will if there are no other alternatives.
Are those plastic boxes made to support pbx wiring?