The one-time pad operates on bits and bytes and is agnostic to what symbols those bytes represent, so, yes, you should be able to use numbers as well.
When it comes to the Vernam cipher the answer is "you should, but for some reason you can't". That reason is most likely the fact that, although Morse code does encode digits, they don't appear to be used with the cipher, as this picture from Wikipedia shows
This next bit isn't the most rigorous answer you'll find but it might do: At some point in Neal Stephenson's Cryptonomicon, the author took the time to describe how the main character, who wants to telegraph
3 using a one-time pad, changed that digit to
THREE. All five letters are then encrypted. It's reasonable to assume Stephenson has done his homework.
Yet the Baudot code used later with Telex machines also supports digits; so the answer to your question Is it possible to use numbers as a key is For some historical reason you can't, and the website reproduces that behaviour.
But that is still conjecture on my part because the author of the tool you tried to use has to say the following:
// One-Time Pad
// This code was written by Tyler Akins and placed in the public domain.
// It would be nice if you left this header intact. http://rumkin.com
// Implements a one-time pad for only alphabetic characters. Preserves
// the character case in the text (not the key).
// encdec = -1 for decode, 1 for encode
// text = the text to encode or decode.
// key = the key (pad) to use