Spell Mastery in items

Can you embed your Spell Mastery skill into an item? For example, you know Pilum of Fire, and have Spell Mastery 3 with it. You want to create a wand that will cast Pilum of Fire, with your Ability of Multiple Casting, so that multiple missiles come out.

Is this strictly forbidden, or could you do it by increasing the magnitude of the effect?

In general you can't do it; in fact, in many cases it's meaningless to even consider it.
For example, Silent or Quiet Casting mastery abilities are either "automatically built into" an item for no additional cost, or not; items do not incur botches, so any casting botch dice reduction is moot; etc.
In other cases it's reasonable to simulate a spell mastery ability by increasing the level of effect. Increased penetration is a standard example, and many but not all Multiple Castings might be simulated by raising the target from T:Ind to T:Group and/or adding one (or more) magnitude(s) to T:Group.
Finally, there are some aspects of Spell Mastery, like Resistance mastery, or Learn from Mistakes, which seem fundamentally impossible to "embed" into an item.

In all cases, however, just as in the related Verditius mystery of Enchanted Casting Tools, what you can enchant in the item has no relationship with what mastery abilities you may know: you are not "transferring" your mastery to the item, you are just "simulating" some mastery in the item.