As written, the rules allow for untrained wielders to roll their Dexterity attribute if they do not have the Lightsaber skill. This makes no sense, as a Lightsaber takes years if not decades to master, and without the Force, the lightsaber is just as, if not more, dangerous to the wielder than it is to the opponant.
To reflect this, the following apply:
When an untrained character attempts to use a lightsaber, they may only roll 1D for their attack roll.
A non-force user may train in lightsaber, but their skill is not based on their Dexterity attribute, though it is listed under Dexterity, it is a specific skill that starts at 1D and costs triple the character points to raise, along with requiring a one month training per pip increase (example: Han is not Force Sensitive and attempts to use Luke's lightsaber in combat, he may only roll 1D to attack, If he spends 1 month and 3CP, he is able to raise his Lightsaber skill to 1D+1).
As always, If a character (Trained or Not) misses the difficulty roll of their attack with their Lightsaber by greater that 10, they injure themselves, and take the full damage of the Lightsaber (usually 5D)