How to return a numpy array or list in a custom function using the eval dataframe method?

I am using python 3.X. I am trying to use the eval()dataframe method including custom functions like this

import pandas as pd
import numpy as np

df = pd.DataFrame({
    'T': [0, 10, 0, 10, 10, 30],
    'P': [0, 0, 1000, 1000, 0, 0],
    'S': [25, 25, 25, 25, 40, 40]
})

def custom(A, B, C):
    # some operations

    aux = pd.DataFrame({
        'A': [0, 10, 0, 10, 10, 30],
    })
    return aux.A      # here is where I want to return the numpy array or list, or dataframe column

eq = 'RES = T + @custom(S, T, P) + 2'
df.eval(eq, engine='numexpr', inplace=True)

But I can only return one float or integer value in the function, a simple value.

So I would like to return a numpy array or a list of values because I want to use the result to operate with the rest of the equation variables. I get this error:

TypeError: unhashable type: 'numpy.ndarray'

Another example:

import pandas as pd
import numpy as np
import seawater as sw
from seawater.library import T90conv

df = pd.DataFrame({
    'T': T90conv([0, 10, 0, 10, 10, 30]),
    'P': [0, 0, 1000, 1000, 0, 0],
    'S': [25, 25, 25, 25, 40, 40]
})

cndr = sw.cndr                  # it returns a numpy array
eq = 'NEW = @cndr(S, T, P)'
df.eval(eq, engine='numexpr', inplace=True)

Is that possible? What kind of types can I return? Is there another way to achieve this?