some-service.ts

import { Injectable } from "@angular/core";
import { Store } from "@ngrx/store";
import { StoreService } from "./store-service.ts"
import { myReducer, IMyState, } from "./reducer.ts"

@Injectable()
export class SomeService {
    constructor(private store: Store<any>, storeService: StoreService) {
        // Add the service specific reducers to the set of existing reducers.
        storeService.addReducers({myReducer});
        
        const store$ = this.store.select<IMyState>("myReducer");
        // ...
    }
}

store-service.ts

import { Injectable } from "@angular/core";
import { Store, combineReducers } from "@ngrx/store";

@Injectable()
export class StoreService {
    private _reducers: any = {};

    constructor(private store: Store<any>) {}

    addReducers(reducers: any) {
        const reducerKeys = Object.keys(reducers);

        for (let i = 0; i < reducerKeys.length; i++) {
            const key = reducerKeys[i];
            this._reducers[key] = reducers[key];
        }

        this.store.replaceReducer(combineReducers(this._reducers));
    }
}

Works like a charm! Thank you.