There are a number of ways to do this, and they all have advantages and disadvantages.
Assume you've loaded your A/R trade opening balance into main account 11000. Now you're trying to load open customer transactions (invoices, credits, unsettled adjustments, etc.). Using a General journal, you pretty much have to use account type Customer, which will automatically use main account 11000 per your system configuration.
One approach is to go ahead and let Customer use main account 11000, and then offset each of those to offset account type Ledger also using main account 11000. Thus, the impact on the main account is 0 as the debit and credit on the same main account net out.
Another approach is to use an alternate posting profile (let's call it Migration) to redirect the main account used by Customer to a dummy account, say your error account. Then, load your opening balances using the Migration posting profile, and never use it again. The obvious disadvantage here is that you have this special posting profile lying around that you hope never gets used again and you can never delete (I think?).
One caveat, be careful to NOT use the invoice field when loading open customer transactions using a general journal. If you do, it will create invoices in the customer invoice register, which will look like sales on your sales reports, and that's almost certainly not right.
The alternate posting profile approach can be used other places too, such as Fixed assets, and it gets worse here. The reason it gets worse is that AX will typically use the original main account when making adjustments (it does this for inventory too, more on that later), so if you've used a dummy main account to load your Fixed asset opening balances, and then later you do a reversal, or transfer, or anything that ignores the current setup and uses the original main account, then the dummy main account will take adjustments long after your migration is complete and the company is under normal operations. This will be irritating to your accountants.
My favorite approach is to load your customer and vendor open transactions, inventory, fixed assets, etc., using the standard posting profiles and rules, and then offset those transactions to a single dummy account. Then, before posting those journals, clear out the A/R trade, A/P trade, inventory balance, fixed asset acquisition and depreciation balance, etc., using a ledger journal and offset to that same dummy account. Once everything is posted, your balance accounts are guaranteed to be correct (because they were zero as you started), and if everything was correct your dummy account will also be zero. If your dummy account is not zero, then you will have to figure out what missed getting loaded, but then you can just load what you missed to zero it. Clean, and simple.
Just some ideas. Hope this helps.