Hi

Is it possible to convert HL or cs mdls (weapons) to quake mdl?

I believe it is possible, but it may be hard to do. I don't know which model format HL/CS use, but i have read somewhere that converting Deathmatch Classic models to quake format is really slow, painful and need a lot of effort to put in to it. DMC and CS are both HL1 mods so probably they have the same model format.
Try to ask ppl on this forum, maybe someone could help you:

http://www.hl-improvement.com/forums/

Some of these guys know nearly everything about GoldSource engine, so i think that someone should know how to convert models from CS to Q1.

it is, i have done this for a mod i did for qw, you can use milkshape

how can I import/export quake mdls with milkshape?

I think :

import : not possible

export : possible

1. download fragmotion full version or trial
2. then go to tools, half-live models
3. and that is all

Check out valve dev info:https://developer.valvesoftware.com/wiki/Category:Modeling

They also list freeware model software that perhaps can do what you want. Certainly a nice page with a wealth of information. And these free apps almost make me wanna dive into modeling myself