It is not possible to add postings to an existing VAT Return that has been submitted.
If the VAT Return has not been submitted, you can remove it, add the missing postings, and then re-create it. If the VAT return is finalized and the values submitted, then it is best to instead include these transactions in the next return you do within the system, but also check if a manual supplementary return is needed based on the value of the missing transactions for the period in question.
If the VAT Return has not been submitted yet, you can remove the VAT Return, post any additional transactions, and re-run the VAT Return.