Custom registration fields to go in a related table

[Fins]

I already have some custom fields that are written to the ps_customer table when a new customer registers and that is working OK.

What I would like to do is have this data, or at least some of it, written to a new related table. So presumably I would have to write the custom fields to the new table, get the new row id and then write this to the customer table as a foreign key.

Could somebody please point me in the write direction on how to do this.