You can import contacts from your Evite account by exporting them to a spreadsheet and then uploading the spreadsheet to Party City.
To do this, go to the Events page and click on the Past Events tab. This will show you a list of all your old invitations. Click the Export Guest List option to download a spreadsheet of the contacts from that event.
You can then upload this spreadsheet file to Party City using the Import Contacts option on the Add Guests page.