Is it possible to put a old hdd without reformatting it in a new build just for data since I'm going to use a ssd as the os? or will I still have to reformat
Depending on the size of the stuff, you could just plug it in and leave it or move the stuff to the SDD, reformat the HDD and then move everything back
Yes you can - I have done it many times. Do not have to reformat - delete the stuff you don't need any more. There may be some permissions you will have to straighten out if it's Win7.
-Bruce