At what point can troops be transferred to a Stronghold?

I have seen this discussed in various threads and different forums, but I haven't seen any definitive answers, just suggested houserules.

From what I can tell, the rules as written do not allow for troops to be transferred from a Noble to a Stronghold.

I understand that Mercenaries can be assigned to a Stronghold when purchased, but can troops assigned to a Noble ever be reassigned TO a Stronghold?