change the ID to the one that is still free (limit up to 510 i think). To check it out, which IDs are still free, go to db and export (by using fifa db master) Stadium table to excel or what for better overview.
Now basically, u have 2 options:
1) open CM15, copy e.g. some generic stadium and let it create with some free ID. Then just rename db name and name, adjust details if you want, assign home team, etc. Then rename all the names of your new stadium files to this new ID and place to sceeneassets to overwrite your newly created generic stadium. Then regenerate.
2) without creating anything in CM15, just directly change the file names to new ID which you want to use and paste it to sceeneassets. Then you need to play a bit with database, as you need to add line to Stadium table, i.e. you need to fully create a new one (most of the items can be just taken from some existing stadium i suppose).
However, you will be still missing a stadium name in game. For this you need to also edit language file in loc folder (see my post above)
and at the end, in order to have the proper stadium -> team linkage, there is a table in db called (i think) stadiumteamassignment which you need to change, as your team would have there linked some generic stadium ID, so you need to change it to your newly added ID.