Looks like there is no way to sync imported contacts. After a little searching in the forums, there may be a workaround. It involves exporting the contacts on your phone to a CSV file then importing that file into your GMail account from your computer.
1. Download the free app ExportContacts by Rishi Srivatsavai from the Market. Run the app and let it export contacts to the default file (/sdcard/exportcontacts.csv). The export should take about 20 seconds for 500 contacts. It will give you the number of contacts exported when finished. Make sure it's 485 or more.
If this app doesn't export ALL 485 contacts, you might try other contact export apps in the market. If none of them export "imported contacts" then sorry, I'm out of ideas.
2. Open you app drawer and open the Files app. Select Phone files and scroll to the bottom of the list. Find exportcontacts.csv and longpress on the file name. Select Share and email the file to yourself.
3. Open your email from your desktop/notebook. Save the csv file to your desktop or and easily remembered location.
4. Open Gmail in your desktop/notebook browser. Select Contacts in the top left frame. Then select Import Contacts... from the contacts menu in the middle left frame. Browse to the location where you saved the exportcontacts.csv file earlier, then select Import.
5. Check your contacts, and make any edits, updates.
6. Go to your Android phone and sync the Google contacts:
Setting / Accounts / Google account / Menu / Sync now.
7. Check to make sure your contacts were synced from your Google account:
Contacts / Menu / Display Group / select Google
The top bar should say Contacts: Google and all 485+ should be there.
8> If you are comfortable that your contacts have synced you can delete the "imported contacts" group.
Contacts / Menu / Display group / select Imported Contacts
Top bar should say Contacts: Imported Contacts
Menu / Edit Group / Menu / Delete Group / confirm with Yes
Let me know if this works. I know the basics work because I tried it with my phone and my secondary GMail account and it worked no problem. I don't have "imported contacts" on my phone since I migrated my contacts to GMail via BitPim before I got the HTC Eris. If Step 1 is unsuccessful at exporting ALL your contacts, there may be one more option. It involves finding the "imported contacts" file which I have heard is on the SD card. But I'm not sure what format the file is, or even if it exists on the SD card.