SAMP objects are contained in the SAMP.img that I remember. What's different from SAMP to MTA is that SAMP use .img archives as well as .col archives. For you to convert SAMP objects you'll have to extract collision models from the .col archive. The naming of collisions is the same as the dff name.
It's important to note that SAMP uses textures for some of its custom models that are already found in the base game, for this reason you'll need to have access to base game .txd files.
You can look up Prineside to reference the model names to make it easier on yourself when converting the assets.
I can convert a handful of walls for you if you cannot figure it out - not very complicated, so please try for yourself and ask questions if you don't catch it. If you're able to learn to convert a wall, you could convert the entire SAMP library yourself.
If you need to know more about MTA modding, please see this thread: https://forum.multitheftauto.com/topic/119240-mta-modding-in-3d/