Yes you can. I accidently deleted them once before I opened the box on a toon, and re-clicked the 'claim button'. It gave me only the box (because the game checks if you have the items already), but I was able to easily regain the DOFFs.
Though I do recommend you keep the DOFFs, if for no other reason than because they are good for DOFFing.
The Cardassian has a bunch of really good traits for the most part, including Resolve, so anything requiring that, with an Armory Officer like he is, is very useful. (Plus if playing an engineer character, you have a free, guaranteed Armory Officer for your Turret kits)
The Romulan is nice for any ship that uses cloaking because he gives a 15 bonus to Aux power while cloaked.
The Bajoran is really awesome, because she is great if you make consoles from Powered Artifacts, since she is a rare level engineering DOFF with Resolve and two crit traits. To compare, she gives an equal bonus to a very rare Engineering DOFF with Resolve and one crit trait.