Grievous was cyborg that was apparently kept alive with a Jedi's blood. As a character build, it wouldn't be too far-fetched to give him the FSEx or FSEm specs so he could take advantage of lightsaber specs fully, if one so chose.
And while firmly ensconced in the Legends category, there are the shards and Iron Knights . The droid PC in your game could discover a shard which then joins him in his body to use him as a host, thus allowing him to become Force-sensitive and taking Force specs.