Recording customer opening balance

Let's review this in parts:

"We have to enter opening balance for each of our customers."

It matters if you are cash or accrual basis. But you want Invoices, not generic Opening Balances.

"I understood if we create a new customer, we can define the opening balance at the same time"

Never.

"but now we have already entered transactions."

Exactly. Opening Balance = I don't know why they owe, or what they owe for. That bypassed Sales, sales taxes, aging, etc. And that creates only Income, when you get paid = uncategorized income. Not sales tax liability, if that is part of your consideration. You are letting the data be generically offset to Equity. If you want Equity, then use Real Functions and Real Equity. And that would be listing a Service or Other Charge item linked to Equity, for an accrual basis entity that already reported this as unpaid sales, income and sales taxes, and just needs to manage AR against Aging.

Once the file is set up, OBE needs to be offset to Real Equity, taken to 0, and it is never used again. Otherwise, you keep making up value out of the air. OBE = I don't know what this is.

"Is there a way to record the opening balance for a client for which we already have entered current transactions (e.g invoices)?"

The Invoice did it, already. That is what causes them to have an Unpaid Sale = AR balance. "Current" transactions and any Open Transactions from your cut-over date, would be entered, to get on track.

For full detail, enter all the open Invoices and Bills as of last year's closing date, as Invoices and Bills dated on the actual date, so that when you run an aging report or statements, they will be correct.  Use an Item called Prior Year or similar and link it to Opening Balance Equity.    You should not use the regular Item linked to income/expense accounts because the offset is Opening Balance Equity.   If you want the actual transaction/s you would go to the prior year's file.  So the Invoices and Bills would create AR and AP against Opening Balance Equity. When you are finished the total AR and AP should agree to the last year's closing balance sheet of course. 

If there is sales tax involved, it may be tricky.

For other balance sheet accounts, except AR and AP (and Inventory, if it is tracked), use the box in the account set-up screen or in the import file, to create the opening balance per last year's closing balance sheet.  The offset will go to the Opening Balance Equity account.

When you are done the Opening Balance Equity account must be zero.

The Opening Balance Equity account is used for just this case of creating a new file for an existing business.