this is really two questions, but may be answered via the first.
Q1: I have a large set of contacts (1400+) in a spreadsheet that already has a column for organisation (or household). ie First name, Last Name, Organisation When I go to import the contacts csv how can I auto-create the organisation contact and have the relationship set up so that an imported contact has the "Member of" relationship. I have looked at https://wiki.civicrm.org/confluence/display/CRMDOC/Importing+Households+and+Household+Members
But i am not 100% clear on the instructions - is it saying to upload the Organisations first, then upload the members of the organisation
Q2: if Q1 is not possible/practical or having imported contacts already that may not have existing relationship data, how can i bulk updated select/filtered contacts to add a relationship in one pass as doing them individually would be a very time consuming and error prone process.