Exact Online support explained me that there is a way, used by portals of large accountants.
The Administrations
topic on XML supports to some extent the creation of companies. It does not matter into which company you upload the XML for a new Exact Online company, it gets added as an independent company.
Code sample to create 100 companies, having a human number 501..600:
insert into UploadXMLTopics
( topic
, payload
, division_code
, orig_system_reference
, orig_system_group
)
select 'Administrations'
, '<?xml version="1.0" encoding="utf-8"?>
<eExact xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="eExact-XML.xsd">
<Administrations>
<Administration number="' || 500+id || '">
<Name>Leeg ' || id || '</Name>
<AddressLine1>Harm Buiterplein ' || id || '</AddressLine1>
<Postcode>2501 TH</Postcode>
<City>Den Haag</City>
<State code="ZH" />
<Country code="NL" />
<Currency code="EUR" />
<Phone>0351448695</Phone>
<Fax>0351448690</Fax>
<Email>info@leeg' || id || '.nl</Email>
<HomePage>www.leeg.nl</HomePage>
<ChamberOfCommerce>12411369</ChamberOfCommerce>
<COCEstablishmentNumber>99</COCEstablishmentNumber>
<ActivitySector code="A" />
<ActivitySubSector code="01" />
<SBICode code="014" />
<CompanySize code="G" />
<BusinessType code="51" />
<StartDate>2015-05-13</StartDate>
<BlockingStatus>0</BlockingStatus>
</Administration>
</Administrations>
</eExact>'
, sdn.code
, 'Create-admin-' || id
, 'Create-admin'
from exactonlinexml..calendar clr
join ( select min(code) code from exactonlinerest..systemdivisions ) sdn
where clr.id between 1 and 100