Contacts can be moved from other devices to your Nexus 7 if those devices support exporting to a vcf file. You can export your "Phone Contacts" or "Outlook Contacts" and then import them into your Nexus 7, but if you do not import them into your Gmail contacts they will not stay synced. The Gmail contacts are stored in the Google cloud, so regardless of the device using the contacts the changes are available on all devices. It does require that you trust Google with your information that is stored in your contacts, but that is up to the individual. Personally I use a paid Exchange service because of the additional security. You can use the manual export/import method, but it becomes a nightmare trying to keep multiple devices synced. Follow these steps to export/import your contacts with an Android device.
Open contacts/people on your Android Phone or device tap menu and select export your contacts will be exported to a vcf file and saved to your sdcard storage area.
Use a file manager like Astro and save the vcf file to Google Drive or Dropbox
Access Google Drive or Dropbox from your Nexus 7 and download the vcf file to the root of your sdcard storage area
Open contacts/people on your Nexus 7 tap menu and choose import and select the account into which you want the contacts to be imported.
Remember continuous synchronization will only happen if you choose a cloud based account like Google and all new contacts are created in that account.
Import and export success varies from device to device using a vcf file, but if all goes well you should have all of your existing contacts transfered to your Nexus 7.
Note that IOS and Outlook can export to vcf files as well.
Edit: Post your results or any additional suggestions and I will update the guide accordingly.