It is not possible to merge multiple Quora accounts together. You will need to manually re-add any answers/posts from the secondary account(s) to the primary account.
Note: it is Quora policy that you can only have one account, so it will be necessary to delete the additional account(s) if you find yourself with more than one account. For details on how to delete an additional account, see “How do I delete my Quora account?” When the secondary account is deleted, all the content from that account will be permanently lost, so make sure that you migrate over the content you want to keep to the primary account.