RIchard
Before I go to far into the detail, is the relationship a oneto one, as if so it really depend on how you import the data and set up the sub account.
For example you can set up you import data with the account information and the sub account information as one record, you then customise the entity record (Account ) in CRM to hold all the data for the primary and the sub account, you create a workflow that on the back of an account create a second account record is created with the sub and contatc information which in workflow will allow you to create the relation to the sub account.
Now if you data is more complecated than that you really need to look at a 90 day licence for scribe its about $200 from memory and will allow you to do all sorts of clever things but remember to get the CRM adapter.
Happy to help out if I can
Alistair