No, you cannot modify the content of a built-in report except for changing date ranges, descriptions, and the like.
But why are you trying to separate expenses from other purchases on the Tax Summary? That seems irrelevant for the purposes of this report, which is designed to support value added tax schemes, under which a purchase is a purchase.
Notice that the heading on the form is “VAT on Expenses and All Other Inputs.” Nowhere does the form separate “expenses” from any other form of purchase. The UAE VAT scheme is a very straightforward implementation of the VAT concept: tax due less tax recoverable equals tax payable (or refundable). You do not submit the Tax Summary to the FTA, so the use of “purchases” in place of “expenses” does not matter at all.
If you want to duplicate the form itself, that would require a report transformation. And that requires some coding skill and is not for faint of heart.
In order to get something similar you will need to assign all of your transaction to a tax code, even those with 0 VAT charge.
You will also need separate tax code per tax return line, e.g. DXB VAT 5%, … Customs VAT 5%, RCM VAT 5%, VAT 0%, VAT exempt … etc.
If you do that, your tax summary report would contain all information required.
However, if you have other kinds of taxes and you want separate reports, or you want to create the exact format as the FTA form you would need to follow @Tut’s advice and create your own report transformation.
You have all the Emirates codes which is correct. They will only appear in your tax report IF and ONLY IF they have been actually used in your transactions.
Moreover, you will need to have those tax codes:
RCM VAT 5% (for import of services under reverse charge mechanism, return items 3 and 10)
Customs Import VAT 5% (Item 6)
VAT 0% (For exports and local 0 valued supplies, item 4)
VAT Exempt (item 5)
You need to use those additional tax codes as well.