How to add data from external drive to iMac?

The mac should be able to READ files from a Windows ntfs or fat disk if there is no encrypting.

System files or files with a specific windows format can not be read, but you do not need those.

You can transfer user fies to the mac with copy/paste in Finder, or you can read them into an application directly from the disk.

If you want the ".dat" files in compressed mail you can use TNEF's Enough from the Appstore.

After you have read in all you want to have in the mac may be you re format the drive on the mac with DiskUtility in mac. Or you keep it as a safety backup.

Welcome to the world of Mac. Some ‘light’ reading to help the transition. I don’t recommend trying to get through all of this at once.

Just move the stuff in My Documents into Documents, the things in My Pictures into Pictures, etc.

If you want to put your pictures into iPhoto, there is an Import to Library command in the File Menu. Use that and point the search to your My Pictures folder on your Windows drive. Similar procedure for Music into iTunes.