how do i add external features to my pipeline?

Instead of get_dummies from pandas use OneHotEncoder from sklearn.preprocessing, together with ColumnTransformer from sklearn.compose. Make a DataFrame with both ‘text’ and ‘category|dummies’ columns as features.

OneHotEncoder expects integers type features. If your features are not to int type, first encode|map them to ints, then apply OneHotEncoder.

# ...
from sklearn.compose import ColumnTransformer
from sklearn.pipeline import Pipeline
from sklearn.impute import SimpleImputer
from sklearn.preprocessing import OneHotEncoder


text_features = ['text']
text_transformer = Pipeline(steps=[
    ('vectorizer', TfidfVectorizer(stop_words="english"))])

categorical_features = ['category']
categorical_transformer = Pipeline(steps=[
    ('imputer', SimpleImputer(strategy='constant', fill_value='missing')),
    ('onehot', OneHotEncoder(handle_unknown='ignore'))])

preprocessor = ColumnTransformer(
    transformers=[
        ('text', text_transformer, text_features),
        ('cat', categorical_transformer, categorical_features)])


pipe =  Pipeline(steps=[('preprocessor', preprocessor),
                   ("ridge", RidgeCV())
])

# ...