For making attacks?
The problem with making a "shield skill" is that in doing so, you are turning a shield into a weapon which can then be used to parry, which can make shield much much more powerful than normal.
Now, Combat Companion does allow for the creation of weapon and shield styles, and there are options in that that allow for shield bashes and for being able to increase shield DB and even retain some of that while making attacks. But these options do not allow for a specific, separate OB for the shield.
If you are wanting to do something similar in RMC, without the styles, then I would suggest something along the following lines:
Shields -- fall into the 1 hand concussion weapon category, uses the Ram/Butt/Bash attack table, size based on shield size (Wall does huge, and you go down 1 step for each size smaller shield), and the shield cannot be used to "parry" anything. Character does not receive Shield DB while making attacks. Perhaps allow a +1 to DB per rank in the skill, up to a number equal the melee DB provided by the shield (i.e. a +20 shield can provide max of +40 to DB when using skill, no DB when using it to attack). Perhaps also allowing 1/2 this additional bonus to DB when making attacks (i.e. when using a +20 shield, 20 ranks gives +10 when using shield to attack, 40 ranks allows +20 DB when atacking (normal shield bonus is lost).
Would that work for you?