Yes, he can subpeona pretty much anyone he desires for questioning or testimony in front of the GJ. But remember, a Grand Jury is not the same as a trial Jury. The GJ does not decide guilt, only if enough evidence exists to charge the subject with a felony. It is a very one sided proceeding where the prosecuto tries to convince the jury to indict.
There is no problem in getting indictments by a DC grand jury on anyone Mueller wishes to charge. It won’t matter if the evidence is flimsy or non-existent. The gold is to indict and then get everyone tied up by trials in the same corrupted DC jury pools, which will easily reach guilty verdicts since the trials are political. Appeal route won’t work, since the District Court is left leaning. Mueller has convictions on anyone he desires in the bag.