Can I use Void Assault on a battlefield that is already a unit, to target that enemy unit and move my unit to that battlefield?

No, you cannot use Void Assault in that manner for two reasons:

1. Targeting Restriction: Void Assault does not target a battlefield; it targets units. The card text is: "Move a friendly unit, then move an enemy unit." You must have a friendly unit to move and an enemy unit to move.
2. Card Mechanics: According to RiftJudge FAQ #9965, because the ability instructions are connected by "then," these are mandatory, sequential actions. You must be able to perform the first action (moving a friendly unit) to initiate the spell.

Regarding your question about using it on a "battlefield that is already a unit"—battlefields are locations and are not themselves units (see Rule 169). If you are looking to move units to a specific battlefield, you would target the units themselves with the spell, not the battlefield, and ensure you have a legal friendly unit to move as the first part of the effect.