3 wire conductor to an electrical subpanel - Can I add a separate conductor for neutral in same conduit?
Nope.  All conductors must be in the same cable or conduit.  NEC 300.3(B).
Further, individual wires are not physically rated to be installed &quot;loose&quot; outside of a conduit or other raceway.  The interior conductors inside 8/2 are also not rated to be used without a sheath.
Lots of people find themselves in this situation, but the simple fact is you need to get the right cables or wires for the job.