Bonuses are saved in item vals so you could use -v* argument of `spawnitem.
Item's Val0-Val4 holds bonus types, Val5-Val9 holds bonus value; if you want to set first bonus you have to set Val0 and Val5, so you use `spawnitem PID -v0 BONUS_ID -v5 BONUS_VALUE; for second bonus you use -v1 and -v6 and so on.
Some bonuses takes BONUS_VALUE as it is, while other only checks for non-zero value; see parameters.cpp for a list of bonuses IDs and logic behind them.
There is at least one bug introduced by Reloaded, connected with `spawnitem and item bonuses, but i'll leave discovering that to you
