Hello @Abdessamad 

Yes, you can use external voucher functionality. Create a new load type connected to voucher type Q and import your voucher through External Voucher Assistant.

The only thing, you cannot delete the voucher lines imported via external voucher. However, you are still able to add new lines in Voucher Postings tab of the Q voucher you uploaded.

Hope this helps

Hello @gumabs,

Thank your for your response, 

Best regards.

Just one remark - usually, you should load separately opening balances of ledger accounts via their dedicated flows (supplier invoices, customer invoices, mixed payments, fixed assets import etc.) and only non-ledger accounts via manual voucher. Dedicated bridge account can be used to tie open items with remaining part of trial balance. 
Therefore, using function group Q is not necesseary, you can use regular M voucher instead.