Clubs from one country can get permission to play in another I believe if
1, There is no-professional league in the country (in the case of Wales, for many years)
2, They are situated on the boarder (in the case of Berwick Rangers)
Gretna (and I think Annan Athletic), played in the English league's
Can name a few clubs who cross border
FC Vduz
Toronto
FC Andorra
and I might be wrong but in the 1970's Wigan Athletic applied to join the Scottish League.
No chance of the old-firm getting permission to move unless every other team in Scotland went amateur or part-time.