I'm pretty sure you can't do what exactly you are asking, but here's what I would do instead.
Make a new expenses account called "Expenses:Unknown" or something like that. Suppose you know that there are $500 of unrecorded expenses in July. Make a transaction dated July 1 sending $500 from Assets:Wallet to Expenses:Unknown. Do the same for each other month (or other "checkpoint" time interval) for which you have missing expenses. At this point your books are already in balance: you know you spent that money, and it will show up in expense totals, you just don't know yet what you spent it on.
Now if you remember that on July 15 you spent $80 on jellybeans, make a transaction dated July 15 sending $80 from Expenses:Unknown to Expenses:Jellybeans. It should appear as a rebate in Expenses:Unknown. (Note that this transaction does not touch Assets:Wallet at all; the money has already been deducted from that account.)
When Expenses:Unknown gets to a running balance of 0, you know you have recorded all the missing transactions. (You can also note that if you get a running balance of 0 just before one of your checkpoints, then you have recorded all the transactions between that checkpoint and the previous one.)
At that point, if you want, you can manually edit all the transactions on Expenses:Unknown to point them back to Assets:Wallet instead. (So your $80 jellybean transaction will now go from Assets:Wallet to Expenses:Jellybeans, bypassing Expenses:Unknown. Your $500 transaction from Assets:Wallet to Expenses:Unknown should be deleted altogether. At the end, you should have no transactions at all that reference Expenses:Unknown and you can delete the account if you want.)
Or you can just leave things as they are: all the Assets and Expenses accounts will have the proper balance, and the only downside is that you won't see your jellybean transaction as a specific debit from Assets:Wallet (you can still see it under Expenses:Jellybeans).