Maybe the best way to accomplish what you want is something like:
Anyone can pick up any shield (how much strength do you really need just to hold it in front of you?)
The shield's requirement minus your shield skill represents a penalty:
* -5% movement speed per point
* -10% shield speed per point
* -10% melee speed per point
* +10% damage taken by shield per point
So anyone can pick up a Huscarl shield but if you have 0 skill in it, then you get all of the above penalties, multiplied by 5 (while you are using it -- no penalty if it's just on your back).
So you'll run 25% slower, the shield will be speed 40 (instead of 80), your weapon will swing 50% slower and the shield will effectively have 50% fewer hit points. Basically you can carry a piece of wood in front of you while you close in on a target but you'll be slower and you'd better put it away before the fight breaks out.
Someone with skill 4 can use the skill 5 huscarl shield, but with x1 of all of the above penalties.