I helped myself. Attachted you can find the rep709-file. Rename it to rep709.php and save it under your '/reporting/'-directory.
The report contains a VAT-declaration for Belgium, but you can adapt it for other countries (symply rename the fieldnumbers of the VAT-declaration).
You need to add the fieldnames of the VAT-declaration in the taxname, ex. 'BTW BE D&G-21/100-M03VT54M49AT64|M82AT59M85VT63'.
When the 'taxname' is used for a transaction, the fields of the VAT-declaration will be populated with the correct values and sums.
So, in this ex., if you make a salesinvoice, the fields M03 en VT54 will be popultated with the salesamount and the taxamount.
If you make a purchase, the fields M82 and AT59 will be populated with the purchase-amount and the 100%-deductable taxamount.
salescreditnote : fields M49 and AT64
purchase-creditnote : fields M85 and VT63
Ofcourse this is not a pretty name of the taxname in invoices/credit notes/deliveries. In these reports, I changed the variable '$tax_type_name' to :
$tax_type_name = substr($tax_item['tax_type_name'],0,3);
So, only the first 3 letters of the taxname are used in invoices/credit notes/deliveries, in above ex. : 'BTW', which is the dutch translation of 'VAT'.
It also works for VAT that is only partialy deductable (ex. 50% deductability of the tax). Read the code. Make a first taxname for the deductable part of the tax (as explained above), and a second taxname for the non-deductable part of the tax, put '_' in front of the taxname for the non-deductable part (ex. _BTW21/50, i.e. 10,5 %, i.e. 1/2e of 21) and FA and the report will do the rest (escaping the non-deductable part of taxamounts in purchases from being part of the VAT-declaration).
I hope this helps someone.