Here are the house rules we're using for after-market additions of weapon, armor, and shield modifiers in DFRPG.
Allowable After-market Modifiers - Climber's, Ornate, Silver Coating.
Allowable After-market Modifiers - Mirrored, Ornate.
Allowable After-market Modifiers - Fine, Orante, Spiked.
Cost for after-market modification is twice the cost of the modifier on the weapon. For example, silver-coating an $80 morningstar will cost $160 (+2 CF) as original manufacture, but $320 (+2 CF, x2) done to an existing weapon. Characters with crafting can attempt their own modifications - but critical failure will waste the money and potentially ruin the piece of equipment (depending on the type of gear and attempted modification.) Modifications done by professional NPCs in town will take 1d weeks but has no chance of failure or ruining the piece of equipment.
This is a frequent request in my game, so I made this ruling. Now it's become a written-out rule, as is the natural progression of such things.