I did it using Gingerbread (Samsung Galaxy Ace 2), but with Ice Cream Sandwich (LG L5) I don't see appropriate action for that.
Previously I entered Contacts (ok), then selected Settings (ok), executed Google Merge (no such option), and Sync Data (disabled all the time).
I see only one available (not disabled) item that is relevant -- configuring sync option for my account, but I understand it will work non-stop, and I want action on demand, just from time to time, when I explicitly select it, not enabling a background sync process.
What also worries me, that in GB I could choose where to store contacts (phone, SIM, google), in ICS after initial import from SIM card to phone, I cannot choose anything.
UPDATE -- (hardcore) WORKAROUND
I found that for given single contact I can switch its type from phone to google, nice, but it does not solve the problem due to the amount of contacts.
UPDATE -- DON'T DO THIS
I tried to somehow do it automatically, so I did (but you don't):
- change the mode of sending vcard to single file
- share contacts
- select all
- type: gmail
- recipient -- yourself
- OK, switch to gmail app
- open the attachment with contacts
- choose target as google account
- all contacts will be merged with existing ones, but now as google ones
And the last step is killing this workaround. Why? Because some contacts will be added (so when you had "A" you will have "A" for phone and "A" for google), the others will be joined (so when you had "A" you will have "A" as before, but now with double type -- phone and google). In short -- the result is mess. Now I have more work to do than manually switching one by one type of the contacts. Geee, progress, right? ;-/