store other files on the time machine external drive

This has been asked before and overall it's a pretty poor idea, why? Simple, all HD's eventually die and many with no prior notice. If you executed this idea, then not only would you lose those files you would lose their backup too. Because External HD's are relatively inexpensive, if you have overflow files you want to store simply store them on another external HD.

You can, but it would be better to use a different external drive. Why, because if and when your current external drive fails, then you will lose both the TM backup and stored files.

If it's important you want a backup, if it's irreplaceable you want more than one backup.