Is it possible to import a Blender model in Source Filmmaker? I can't find out how. If so, is there a way to do it without downloading anything?
Importing from blender to source is a full process, and there are plenty of tools to make it easier, all of which are free. technically it's possible to do it all only by installing blender source tools (https://steamreview.org/BlenderSourceTools/), an addon to export as DMX or SMD and use the stock tools provided by valve, that are in the game/bin folder. But everyone uses community built tools, crowbar and VTFedit at least.
with your model created, export it as smd or dmx with the BST. the material you set will not be exported, but it's important you use the same name when you create your materials.
You'll then have to write a QC file, it's a conductor, indicating to source how you want it to behave in game. Naming the object, assigning it models and animations etc. (https://developer.valvesoftware.com/wiki/QC)
You'll also have to create your materials(https://developer.valvesoftware.com/wiki/Material). they can be as simple as a single color or an advanced material with light and reflections. it's important that you use the VertexLitGeneric shader for props. the materials are saved as VMTs and the location they should be placed is the one set in the QC, relative to the Game/game/materials folder. A VMT assigns a role to each texture in your material, the simplest being $basetexture "" (a basic albedo map) those textures should be converted into VTFs with the tool VTFedit and are conventionally placed in the same folder as the VMT (http://nemesis.thewavelength.net/index.php?c=178)
once all your assets are created you'll want to install crowbar, the tool for importing into source, (https://steamcommunity.com/groups/CrowbarTool) and set it up for your exports. selecting your QC file and compiling it will tell you if there's an error, if there isn't it'll let your "use in view" then "view" to open your model in the HalfLifeModelViewer to make sure evrything is working. If it is you can open SFM and find your model.
https://developer.valvesoftware.com/wiki/Main_Page this site will be very usefu in looking up how to use any of the tools, commands and files.