How can I put custom props into my map? Like ones downloaded and not official from any source games?
Can you be more specific on that question?
Possible answer to your question:
-To add props into Source SDK, you have to first export the object or the prop from a 3D modeling programs using SMD format, then using notepad++ or any program language tools to compile the texture and the model into .MDL format. From there, it should be available in Hammer or in Model Viewer.
-To ensure the prop is in your .bsp files of your map, use Pakrat and embed both the models and the textures to your BSP and be sure to save it.
-To View the models in Hammer, create a entity and change the class to prop_static. From there, go to the world model, and find your model. Once you found your model, hit ok or enter, and press apply. Your model should be visible in hammer.
Hopes that help!