Yes, that's pretty easy.
You'll need a new column to add up the total amount bet as open-raises:
amt_p_open_raise_ttl_bb =
sum( if[holdem_hand_player_statistics.flg_p_first_raise AND holdem_hand_player_statistics.flg_p_open, (holdem_hand_player_detail.amt_p_raise_made / holdem_limit.amt_bb), 0])
..then we can create a new stat to divide that column by the number of open raises, which is the built-in column cnt_p_raise_first_in, so the stat expression would be:
amt_p_open_raise_ttl_bb / cnt_p_raise_first_in
Tutorial - Custom Reports and Statistics