Adding a secondary data file will not break mirroring as long as you have the same folder structure on the mirror server.

For example adding datafile2.ndf:

Add datafile2.ndf to C:\Data\ on primary and C:\Data\ exists on mirror everything is fine.

Add datafile2.ndf to C:\Data\ on primary and C:\Data\ does not exist on mirror, then mirroring will be suspended until the directory is created on the mirror side, then you will have to manually resume mirroring.