You want to use
Silent 3Ditor (S3D). You would open the AssetName.dat files that you would find in either the Air, Sea or Submarine folders. A playable submarine is in the Data \Submarine \SubName folder, ships are in the Data \Sea \ShipName folder, and airplanes are in Data \Air \AirplaneName folder. The names do not always match the platform though, so you might need to do some digging first. The dat files though hold the 3D parts of the basic platform, less its guns, periscopes, and other sundry equipment. You do have to look through all of the various nodes, since there are several sections that might hold 3D parts, and you'll need to export each of those parts.
You'll see textures listed above (numerically) the main pieces of the model, and then you might notice that the main node that holds the model, looks to be a "child" node of the node above, such that you'll see like Node 6, 5 and then 7 listed, and Node 5, listed under node 6 is what holds the main part of the model. Using the Gato in FotRSU as an example, the file to open in S3D is "Game \Data \Submarine \NSS_Gato \NSS_Gato.dat". Once that is open, the main part of the Gato hull model there is under Node 7, with 8 as the "parent" above it. Node 9 is the "label" for Node 8. Most other types of parent/child set-ups are the Parent is 7, the child 8, but not with the 3D models. Those are actually "linked" by the "Model Id" on the parent page. Anyway, on the 3D Model page of Node 7 is a blue text link for "Import / Export", and you would click on the "Export" link. From that, a dialog opens with a Windows file manager to help you find the path where you want to export the model to, and the format. That will default to the last directory you used in any other Windows dialog, and will have the "WaveFront Object" as the file format. You can choose Collada dae, if you find that easier to work with. I have not used that format, and am not qualified one way or the other to advise.
When using Blender for the obj format, you use the "Import" in Blender, and the model will come in standing on its tail. That is just the format that is used in S3D. You can of course, rotate things as you need, to be able to work on them, but definitely put it back in the orientation it was originally prior to exporting. After you do your edits and Save yourself a Blender file. If you export to obj format again, you do have to set a few settings in the Blender Export dialog beforehand, so that S3D can import the obj back in. Depending upon the model, you might have to check the "OBJ Groups" button, and down in the Geometry section, you'll want to tick "Triangulate faces" at a minimum. Dependent upon the model, you might have to make more choices. You can Save your settings as an "Operator Preset" by clicking the "Plus" + sign in the upper right of the dialog, and naming your preset.
Unfortunately, there are no comprehensive tutorials on S3D's use, but by the same token, there is a myriad amount on Blender, and that doesn't help me much either. I get a few ideas here and there, but the school of hard knocks is a tough school master, so crash a few programs, waste a few hours with experiments, and it actually isn't a waste when you learn... Here is skwasjer's REL post for
S3D - Silent 3ditor
There are a few other postings from him and others on the subject, but unfortunately, his original web page is no longer live, and most of the tutorials he lists are no longer available. The main thing to remember is "Have Fun"!!! lol -
PS: There is nothing wrong with sharing what you do, so long as you attempt to contact the original modder of whose work you are basing your work on, several times, trying to gain permission, and giving that original author full credit for their work.