No probs
A potentiometer has 3 terminals : the 'start' (of resistance track), the 'wiper' (variable resistance) and the 'end' (of resistance track).
You can use all 3 terminals, with the 'start' terminal as the 'hot' signal, the 'wiper' terminal as the 'attenuated' signal and the 'end' terminal often goes to ground.
You can also use it as a 2 terminal device, where the 'wiper' terminal can be connected to either the 'start' or 'end' terminals.
In the case of the 'Output Level' potentiometer before the b340 line amp module, you want to connect a pot, probably 10K log taper law (not reverse!) as a 3 terminal device.
ie. pot 'Start' terminal goes to pad 18, 'Wiper' terminal goes to pad 19, 'End' terminal goes to audio ground.
So that is operating as a 'shunt-to-ground' fader across the output of the ba338 and providing attenuated signal to the b340.
Hope that clears it up for you.
The 'Gain' rotary switch, in front of the ba338 is a little different - here it operates as a 2-terminal device : the variable resistance in the '-ve' leg of an opamp wired into a non-inverting variable gain stage.
So you would need a 10Kohm pot, reverse log taper 'law' with the 'wiper' and 'end' terminals connected together and going to ground thru a cap.
Check the api style mic pre doa '-ve' leg and you'll see what I mean.
Furthermore, you want to emulate that pot as a rotary switch of 12 steps : so you need to wire up a single deck, make-before-break switch, say a Grayhill to emulate the 10Kohm reverse log taper pot.
AGain, you can see exactly how this is done from an api mic pre doa.
Which is why I was asking 'how historical do you want to go' ? You are basically doing an api mic pre using a ba338 module as a 'discrete op amp non-inverting gain stage' with an additional b340 class AB line amp + traffo backend.
As far as eagle footprints and lib components and hat not, you are essentially asking if anyone has a api mic pre project done in eagle that you could cut-n-paste.
Probably there is - spome more research and asking nicely I suppose would give you an answer. I would say however, if you are going to do that, then why not simply get an already fabricated pcb?
If you are more of a 'recording' guy than a 'electronics' guy, I would recommend getting a good kit with pcb+parts. There's heaps of them available.
You still have the thrill of diy, at a reasonable price, and wind up with a really nice product. You'll probably wind up with a more 'focussed' end result too ie. a distinctive 'neve' sound or a distinctive 'api' sound. ... rather than something in-between, which may wind up being very good performer but without any real uniqueness ... kinda.
However, if you are already diy fevered to the max, go for it and fab your own pcb!
But anyways, it's all good.