Dude the top pinned post in the mod forum is titled:
[Tutorial] Modeling, texturing and importing models into Sins...
You need to read through it but the long and short of it is download the XSI softimage mod tool, import your OBJ files, do some conversion/texture work save them as an XSI model, then convert them with the mod tool and import them into SoaSE to see the results. Then sort out the weapons and effects....
Once you have learned how to basically use XSI you should be prepared to spend 10+hours (significantly longer in some cases) per ship model to do the conversion, so if you are asking someone to do it for you
, good luck. Your best option is to do one yourself and show/sell the idea so if people buy into it you may get some offers of help.
Also a word of caution, if you are using other peoples models the onus is on you to get permission or follow the conditions of usage, a good example is modellers don't often like their models used out of context e.g. USS Enterprise fighting a Star Destroyer.