We can add data files to mirrored database without break mirroring, in otherwords no need to reconfigure whole mirroring.
In this post we will see the steps
1. Make sure database mirroring is synchronized state
2. Disable Logbackup jobs if any
3. Remove existing mirror server from mirroring
4. Add secondary file or filegroup to database on principal server
5. Generate transaction log backup and restore it on mirror server with MOVE option
6. Reinitiate the mirroring
This will save a lot of time because we are not breaking the mirror completely and it is very much helpful while we are dealing with large database.