If you're missing something, so am I.
While there's a mechanic for decreasing the Addiction Threshold (it drops if you avoid use for long enough), there seems to be no mechanic for increasing the Addiction Threshold. And a Threshold of 2 is trivial for most Awakened, who tend to max out Willpower anyway. Things can happen to decrease the relevant attributes or give other penalties to the test, but they seem to be rare unless you started the game already focus-addicted.
By the Missions FAQ, since addiction tests represent permanent character effects, they must be done using the "buying hits" rule (as you've suggested doing anyway). If I ran into this in a home game, I might make the player roll instead of buying hits.
Or you could make the case as a GM that a character who runs around with 10 Force of active foci all the time is already addicted and is just in denial. Runners will claim it's for "operational purposes" and justify it all they need to, but...
(Most PC mages I see at the table run around with their Force 2-3 Power Foci always on. Of course, none of this is mechanical, just an RP consideration.)
