Time Machine after Reinstalling HD

I upgraded to Yosemite, and my iMac started going slow. There was some sort of kernal_task. Long story short I gave up trying to fix it and I decided to erase the Mac HD and reinstall Yosemite. Everything is fine, except now I want to add some files and folders back to the Mac HD from Time Machine and I want to reactivate Time machine. My question is, if I activate Time machine to restore a bunch of files, will time machine reformat the external HD thus losing the contents?

Possibly a stupid question but I never had to restore from time machine.

Thanks,