To insert a new model use Spirit, ask Vassy, think he uses it... It?s some sort of HL port with added stuff, new entities, multiple entities in a single brush... Don?t know, never used it as it ain?t compatible with classic HL (if you use this new stuff).
You can also get a Pak viewer and change the grunt model for the Barney model. This could work altough grunts have at least 3 or 4 different skins/models and Barneys just one.
To make new weapons and new action you must code it... If you know C download the SDK (or something like that, I have it, but stored in CD until I learn C) and get to work... See if with Scripted sequences you can make the monster to do what you want (if it?s simple and doesn?t happen to often).
You can show a model with the monster_generic entity but it will only respond to Scripted Sequences and, well, it doesn?t does much.