All previous entries have been imported across already. Unknown users will have been given an OpenID over there. I believe you can use this or, if you have a dw account, you can 'claim' the openid. See https://www.dreamwidth.org/support/faqbrowse?faqid=84 for more details.
Also, I have added a Giles_Shorts Collection on AO3 - http://archiveofourown.org/collections/Giles_Shorts - if you are posting there and wish to link to us. I'm happy for anyone to post in either (or both) DW and AO3, though the prompts will only be on DW for now.
Hopefully I will see you all on the flip side.