from tkinter import *
from tkinter.ttk import *

class Window(Frame):
    def __init__(self, master=None):
        Frame.__init__(self, master)
        self.master = master

root = Tk()
app = Window(root)


def add_base():
    out = []
    for x in combo['values']:
        out.append(int(x))
    out.append(int(txt.get()))
    combo['values'] = sorted(out)
# set window title
root.wm_title("Numerical converter")
lbl = Label(root, text="Please enter a number!", font=("Arial Bold", 25))
lbl.grid(column=0, row=0)

btn = Button(root, text="Add", command=add_base)
btn.grid(column=0, row=4)

txt = Entry(root,width=10, state='enabled')
txt.grid(column=0, row = 1)

combo = Combobox(root)
combo['values']= (2, 10, 16)

combo.current(1) #set the selected item

combo.grid(column=0, row=3)

root.geometry('400x300')
root.mainloop()