Nothing wrong with adding the backstab skill if you want. If however your main concern is the damage being done I would start looking into the custom equipment section. For example the Dagger listed on page 33 does D4 damage and cost 1 HP. If you move into custom gear on page 35 you can spend 3 hero points and get a weapon that does D6. This weapon can still be a dagger the player can come up with a reason for why it does more damage than the standard 1HP dagger. Or if you just want to stay with the standard 1HP dagger there is already the option to add an extra die of damage for 3 HP by customizing it to make it deadly for 3HP as seen on page 36. So for 4 HP you could get a deadly dagger that does 2D4 damage or for 6 HP you could get a custom deadly dagger that does 2d6 damage.
All that said if you want to add in a Backstab skill I wouldn’t put any of the limitations on it you mentioned. Since if it is a skill they would already have to roll under the skill to even use it. So if they invest the 3HP minimum to buy the skill they will only be using it 15% of the time, and as the GM you can always just make the backstab roll more difficult to simulate the need of the character to have the strike go unnoticed and to get in to place. Then the player makes the roll to see if the backstab is effective. If the skill fails I would just allow the normal weapon damage.
Another option to look at is to use Core Abilities (page 29) with a rename. There is Battlemaster that is written as if the character is a soldier but the mechanics are similar to backstab in that they allow for more damage on attacks. You need to roll max on the damage die and if you are rolling a d4 with the dagger it is more likely to explode than when using weapons that roll larger damage dice. And if you use a deadly dagger (2d4 damage) with battlemaster there is a 44% chance you will roll a 4 on one of those 2 dice.
If you want something more consistent but with a cost of a lower top end on the damage you could rework the “Uncanny Shot” core ability to work with daggesr rather than with ranged weapons. Then every attack from the rogue while using a dagger has an extra damage die and will ignore 1 DEF. So when they are using that 1HP dagger they now do 2d4 damage and they ignore 1DEF on the target. Move up to that deadly dagger and they are now doing 3d4 damage and ignoring 1DEF. And remember they can always get that custom deadly dagger and start doing 3d6 damage.
