Basically I have shed loads of spare core 2 duo slim line desktops, is it possible to utilise all of them to create a massive freenas cluster so i can use all the storage on them?
No.
You could probably look into NetBSD and hammerfs. Old desktops can be unreliable. Clustering them does not make them more reliable it just makes it more likely that something will catastrophically fail.
If you want to go that route with your desktops I believe you can go with any standard linux install and glusterfs. But no, can't do anything like that with FreeNAS.