Question about Half-Life maps. Thread

Evening mentlegen.

I'm been fiddling around with Half-Life, and I've been thinking about adding a few tweaks to maps from the main campaign. Like a few more decals and monsters here and there, Any way I can pull this off and have a playable map in the end, with the edits? I've heard mixed opinions about decompiling a map and going from the decompiled .map file, not sure what to do. Also, are the .rmf files for Half-Life available to the public?

Any tips would be appreciated.