1 Tb Hard Drive not adding files

I have a WD My Passport 1 Tb external drive.  It is currently formatted for Window (NTFS).  There is info currently on the hard drive.  Can I change the formatting to MS-DOS with the files on it, or will it erase them?  I was a PC user and now I have a MacBook Air and want to use the same hard drive, I just don’t want to reformat it and lose all the info on it already.  (It holds all the family photos and a lot of music).  

Thanks in advance! 

No, changing the format of a drive (NTFS, FAT32, ext4 etc) will erase the drive.

As default OSX cannot read NTFS drives, but from a quick search there are ways to allow it to be done. See for example the link below:

http://www.cnet.com/uk/news/how-to-manually-enable-ntfs-read-and-write-in-os-x/

I don’t own a Mac so I can’t test it, but it may be helpful to you. The above link was just the first one that came up - do your own search for reading NTFS drives on a Mac and you can find other variants or pieces of software that may help.