How to add additional observations to panel data?
Not sure if this is on-topic, but I'll give a simple solution. It's perhaps not the most efficient, but should be easy enough to follow.
Assuming you also want to keep &quot;category&quot; and not just &quot;id&quot;, how about something like the following:
use sample.dta, clear
tempfile month10
preserve
keep if month==8 | month==9
collapse value , by(year category id)
gen month=10
save `month10' , replace
restore
append using `month10'