I'm not sure about that. I think the forum administrators don't want people converting maps directly. I don't think they care if you use any models or textures from other games. The reason why is the fact that in recent years, I've seen mods released on here that use models from non Rockstar games, and they even say what game the models are from.
Evidently, conversions from anything before 2005 are fine, anything after can consist only of props, which buildings are not.
What if you were to convert from The Crew Beta? That was technically free and the content isn't considered to be from the final game. On the other hand, I could of swore I saw things from 2005 and after. For one, VCS is 2006.
Also if anything were to be converted from The Crew, there are fences around many things that prevent you from going to certain parts of the map. Especially in cities. Removing those would change the model. I thought you could do it if you altered the model. But you would think that they wouldn't care about buildings, as long as they are being used on a map that is different from the original.
Say for example, you use Driver 2's Chicago for Midway, Driver 3's Miami for VC, SA's San Fierro for SF, SA's Las Venturas for LV, along with GTA IV's map and GTA V's map. Then for Midway, VC, SF, and LV, you replace landmarks with the corresponding higher quality ones from The Crew. Then you remodel stuff that isn't HD. You could also alter the maps as well. On Driver 2's map, I think there are a lot of areas where streets could be added in. For example, if you look along the river on the map, there are a lot of bridges that don't have roads beyond them.
The reason why I recommend that X-Seti should use stuff from other games would include:
1. Making all of this is a lot of work for one person. However, I don't see him quitting anytime soon. This is probably the only mod that I know of that is around today, and was around back when I first started modding GTA, and was around before that as well.
2. The Crew has very good, detailed, and accurate models (with the exception of names being changed). If they are used, he will be able to focus his effort onto another part of the map which would need work. That is, remodeling things that aren't in The Crew, but are in Driver. For example, I think both Watch Dogs, and The Crew don't have the Daily Center, where as Driver 2's map does. He could also put work into the country side between the cities. Also, in the country side, he could have land marks from The Crew, like Mt Rushmore, Kennedy Space Center, etc...
3. If X-Seti didn't use anything from The Crew, he would model the same land mark. He would have took time to make the same exact building, when he could of been working on another part of the map. This means that X-Seti would have more time to make a more detailed, and possibly larger map. It also means that he would progress in creating the map faster. It also means that he would have more time to focus on optimizing the map as well.
Anyways, the rules on here have gotten more flexible. I remember my EFLC Map Updates in GTA IV mod got shut down. Now, we got mods converting GTA IV's map to SA, GTA V's map to IV and SA, etc... (But PS: I sure hope that The Crew's map on GTA V will happen. I don't care what anyone says, because that would be practically an open world orgasm on a hard drive, and if it were to be put on a custom MP server with like a 100+ players, it would probably be one of the most fun multiplyer experiences ever.)