Or rather, could they have modified Skylab B to be able to dock with Skylab A? Possible, but very difficult.
The Apollo/Skylab docking system was an asymmetric probe-and-cone setup, with the active vehicle (with the probe) flying into the passive vehicle (with the cone). <insert dirty joke here

> Skylab's Multiple Docking Adapter (MDA) had two docking cones, one for usual use (forward) and one for rescue (nadir). So, without modification, two MDAs could not dock together.
To do that you could put a probe on the end of the Skylab B MDA, but that would have severe clearance issues, require the removal of B's Apollo Telescope Mount (ATM), and leave only 2 free docking ports, with no free room for rescue (assuming 3-man Apollos).
More plausible would be to replace the ATM on B (which launched forward of the MDA anyway) with a interface module that provides a probe for docking to Skylab A, two passive cones for Apollo CSM docking, and teleoperation equipment so that a crew in an observing CSM can remotely guide the docking.
Honestly though, at that point, you really should just launch a proper 10-meter station, rather than trying to jury-rig Skylab B. See here, for example:
http://beyondapollo.blogspot.com/2010/06/mcdonnell-douglas-phase-b-space-station.html