I am VERY familiar with the Springfield Armory XD magazine procedure.
The Walther P99 .40 Auto magazine body will accomodate 14 rounds of 9mm ammunition. Without fairly substantial feedlip adjustment and a 9mm magazine follower, it is my opinion that the same procedure enjoyed by XD owners is not feasible with the P99s; I have "toyed" with the possibility.
Please remember that modifying the .40 Auto P99 magazine to function properly in a 9mm P99 would cause this magazine to (more than likely) become useless in the gun for which it was originally intended; THIS IS ILLEGAL.
In contrast, the slight adjustment required with the .40 Auto XD magazines to function perfectly in both caliber XDs, apparently, is not illegal. Please be advised that I am not qualified to render legal advice.