Ich habe mich heute einmal ausgiebig mit diesem Problem beschäftigt und es tatsächlich zum laufen bekommen. Da ich nirgends eine Anleitung dazu fand, schrieb ich es mal auf meiner Newsseite zusammen. Hier eine Kopie der Anleitung, damit sie nicht so schnell in den Untiefen des Internet verloren geht.

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Okay, here I tell you now step by step what I did to get Hammer to work with HLS and also get shot by a big turret.
1. Download the official Half-Life: Source fgd here: http://developer.valvesoftware.com/wiki/Half-Life_Source.fgd
2. Edit that fgd, search for monster_miniturret, copy that part and make a monster_turret (by simply removing mini in the entity class, model, and so on) -- I don't know why Valve left that out...
3. Start Hammer with any game... maybe it's best to do that with "EP1/Half-Life 2" in your SDK target platform selection.
4. In Hammer, go to Tools/Options/Configuration, Edit the configurations and add a new one, name it HLS or something along those lines.
5. Make sure the paths in that configuration match those where your HLS is installed, and add the modified .fgd-File you have.
6. Close Hammer and Steam altogether and find the GameInfo.txt in your HLS\hl1 directory.
7. In that text file, set the application ID at the top to 360 (HL1SDM) or 280 (HL1S), I can confirm that 360 works well. If you omit this step, all Hammer viewports will be all black.
8. Start Hammer with your new config selected in the SDK platform selection dropdown, make a room and add a monster_turret, set its autostart flag to true. Note that you can use the HL1 textures by entering "halflife" in the texture window.
9. Compile the map from the Hammer dialog, but do not run the game from there. Instead, load the map from the game's console.

Shrinker @ Beyond-Veils, ScottBaker.eu (mirror)
http://www.thewall.de/forum/thread/hammer-fuer-half-life-1-source/63912.1.html

Wenn Zeit besteht kann das entsprechend in ausfuehrlich ins wiki uebertragen werden.

There is nothing wrong with high standards. It's your problem that you don't meet them.
If you think it's simple, then you have misunderstood the problem.
When a customer says "nothing has changed", assume they're lying.

bis jetzt gibt es ja scheinbar noch gar nichts [mehr] über hl1:source im wiki.

@Shrinker: ich hab zwar kein hl:source, hab mir aber trotzdem das tutorial durchgelesen. Sehr klar beschrieben und kein Geschwafel. Danke!

Resetting politics:
http://www.onlinepartei.eu

http://www.thewall.de/forum/thread/half-life-source-entitiesbeschreibungen-ueberhaupt-noetig/62113.1.html

There is nothing wrong with high standards. It's your problem that you don't meet them.
If you think it's simple, then you have misunderstood the problem.
When a customer says "nothing has changed", assume they're lying.