Importing a QFX file into a specific account however creates another account

Is the account you want to import into currently activated for downloading using One Step Update?
If so, creating a (temporary) new account register would be the only alternative, unless you want to deactivate the account register before importing.

If you have the new temp account register already created, you can (relatively) easily move the transactions which you need from temp to original register:
In the temp register remember to never move the Opening Balance transaction. Block-select the transactions you need (similar to selecting files with Windows File Explorer, e.g., click first transaction, scroll down to last transaction needed, shift-click last transaction). Right-click one of the now-highlighted transactions and select "Move transactions" from the popup. An account selection list appears. Pick the desired original account from the list. The transactions will now appear there.
When you're done moving transactions you can delete the temporary account register.

The Online Balance is downloaded from the bank, as of the last One Step Update download.
You just imported a bunch of transactions to backfill your register with older transactions. Now you've got some cleanup to do.
Locate the Opening Balance transaction in your register and set it to the desired starting date and what the account balance should be, as per the bank statement on that date, e.g. 12/31/2022
The opening Balance transaction must be the oldest transaction, by date, in your register.

Now compare your register to printed bank statements, add missing transactions, delete duplicates, change Payee Names to ones you'd like to use vs. what was downloaded, categorize transactions as needed, verify amounts are correct.

Going forward one month, one bank statement at a time, you should be able to match the register balance to the bank statement's month end balance until you reach today's transactions.