*gasp!* I ALREADY ANSWERED THAT!

Do you mean adding new statpanels, or adding new items to a statpanel? Both are possible. Please give more details about what you want to happen. Will there be multiple things added, or is it just one or two items in the course of the game?

You could create a list for your expandable stats.

mob
var
list/ExpandedStats = list() // list for expanded stats

Stat()
// add this to your Stat() proc to display your expanded stats
for(var/S in ExpandedStats)
stat("[S]:",ExpandedStats[S])


To set a stat, you just use:
ExpandedStats[statname] = value

For example, if you study a book and it should grant a rating of 3 in "Dwarven Literature of Barsaive", you enter
ExpandedStats["Dwarven Literature of Barsaive"] = 3

With the Stat() code above, it would display this in the statpanel:
<font color=red>Dwarven Literature of Barsaive:</font> 3

To see what your rating is "Dwarven Literature of Barsaive" is:
var/skillrating = ExpandedStats["Dwarven Literature of Barsaive"]

To increase the rating:
var/skillrating = ExpandedStats[statname]
ExpandedStats[statname] = skillrating + amount_of_increase