The only problem I think you might run into is finding models. I've never seen a Freespace mod for any other game, and don't know how I'd come up with them, besides making them on my own.
As has been pointed out, the HW2 freespace mod does exist. The models used in that are straight rips from freespace 2 itself (not the SCP, but the original FS2). I should know, I worked on putting a few of the ships into that mod. Just wanted to say that the models already exist, and that they can be taken straight from the game and converted into any format that is required. If the original models are high enough quality for you, I'm sure the guys at the SCP will lend you their high-poly ones, but I don't think that would go very well performance wise.
Actually, I think i'll post a short tut, in case anyone wants to pick it up (at the moment I don't have the enthusiasm/time to get into modding another game, but anyway).
Firstly, you will obviously need a copy of freespace 2. This can be obtained from a couple of places on the net for free. Now all the models that FS2 uses are packed into .VP files. Using a program like ModelView32, you can open up these .VPs and you will get a look at the models contained within. Using this, you can note the names that each model has (for example, the Orion model is called capital01) and what textures it uses (also contained in the .VP). Now with VPVIEW32, you can open the .VPs and actually extract the files (a model file .POF, and various texture files) that you need. The textures can be converted to DDS (I assume that is what Sins uses) with something like photoshop, but the .POF is a bit more complex. There is a program called POF Construction Suite that can convert these .POFs to .COBs. Now that we have a .COB, I used Deep Exploration to open that .COB and save it to my file type of choice. (Once again, no idea what sins uses, but no doubt you could get it to the proper format from most of the common filetypes with sometype of tool). Granted most people will not have a spare copy of Deep Exploration lying around, and in that case I don't know of any other program that will open it and convert it to whatever you want. All I can say that a .COB is used by Calgari Truespace modelling software, and I think there is a trial available, so perhaps you could use that.
Phew, sorry for the lengthy post, but I just wanted to point out the models are out there, ready to be used.